Fix #74

Add more property aliases and do them properly (no more lambdas and
string lookups) which means we can remove `_alias`. This commit also
defines `__slots__` for all classes which should prevent assignation of
invalid attributes with an AttributeError (more friendly than silently
doing the wrong thing). Finally, it cleans up all the property defs to
use Ben's preferred decorator style.

gpiozero/output_devices.py

@@ -25,6 +25,9 @@ class OutputDevice(GPIODevice):
         `False`, the `on` method will set the GPIO to LOW (the `off` method
         always does the opposite).
     """
+
+    __slots__ = ('_active_high')
+
     def __init__(self, pin=None, active_high=True):
         self._active_high = active_high
         super(OutputDevice, self).__init__(pin)
@@ -83,6 +86,9 @@ class DigitalOutputDevice(OutputDevice):
     optional background thread to handle toggling the device state without
     further interaction.
     """
+
+    __slots__ = ('_blink_thread', '_lock')
+
     def __init__(self, pin=None, active_high=True):
         super(DigitalOutputDevice, self).__init__(pin, active_high)
         self._blink_thread = None
@@ -165,7 +171,9 @@ class LED(DigitalOutputDevice):
     anode (long leg) of the LED, and the cathode (short leg) to ground, with
     an optional resistor to prevent the LED from burning out.
     """
-    pass
+    __slots__ = ()
+
+LED.is_lit = LED.is_active
 
 
 class Buzzer(DigitalOutputDevice):
@@ -175,13 +183,16 @@ class Buzzer(DigitalOutputDevice):
     A typical configuration of such a device is to connect a GPIO pin to the
     anode (long leg) of the buzzer, and the cathode (short leg) to ground.
     """
-    pass
+    __slots__ = ()
 
 
 class PWMOutputDevice(DigitalOutputDevice):
     """
     Generic Output device configured for PWM (Pulse-Width Modulation).
     """
+
+    __slots__ = ('_pwm', '_frequency', '_value')
+
     def __init__(self, pin=None, frequency=100):
         self._pwm = None
         super(PWMOutputDevice, self).__init__(pin)
@@ -214,35 +225,35 @@ class PWMOutputDevice(DigitalOutputDevice):
         self._pwm.ChangeDutyCycle(value * 100)
         self._value = value
 
-    def _get_value(self):
-        return self._read()
-
-    def _set_value(self, value):
-        self._stop_blink()
-        self._write(value)
-
-    value = property(_get_value, _set_value, doc="""\
+    @property
+    def value(self):
+        """
         The duty cycle of the PWM device. 0.0 is off, 1.0 is fully on. Values
         in between may be specified for varying levels of power in the device.
         """
-    )
+        return self._read()
+
+    @value.setter
+    def value(self, value):
+        self._stop_blink()
+        self._write(value)
 
     @property
     def is_active(self):
         return self.value > 0.0
 
-    def _get_frequency(self):
-        return self._frequency
-
-    def _set_frequency(self, value):
-        self._pwm.ChangeFrequency(value)
-        self._frequency = value
-
-    frequency = property(_get_frequency, _set_frequency, doc="""\
+    @property
+    def frequency(self):
+        """
         The frequency of the pulses used with the PWM device, in Hz. The
         default is 100.
         """
-    )
+        return self._frequency
+
+    @frequency.setter
+    def frequency(self, value):
+        self._pwm.ChangeFrequency(value)
+        self._frequency = value
 
 
 def _led_property(index, doc=None):
@@ -265,6 +276,9 @@ class RGBLED(object):
     blue: `None`
         The GPIO pin that controls the blue component of the RGB LED.
     """
+
+    __slots__ = ('_leds')
+
     def __init__(self, red=None, green=None, blue=None):
         self._leds = tuple(PWMOutputDevice(pin) for pin in (red, green, blue))
 
@@ -274,6 +288,13 @@ class RGBLED(object):
 
     @property
     def color(self):
+        """
+        Set the color of the LED from an RGB 3-tuple of `(red, green, blue)`
+        where each value between 0 and 1.
+
+        For example, purple would be `(1, 0, 1)` and yellow would be `(1, 1,
+        0)`, while orange would be `(1, 0.5, 0)`.
+        """
         return (self.red, self.green, self.blue)
 
     @color.setter
@@ -282,13 +303,15 @@ class RGBLED(object):
 
     def on(self):
         """
-        Turn the device on
+        Turn the device on. This equivalent to setting the device color to
+        white `(1, 1, 1)`.
         """
         self.color = (1, 1, 1)
 
     def off(self):
         """
-        Turn the device off
+        Turn the device off. This is equivalent to setting the device color
+        to black `(0, 0, 0)`.
         """
         self.color = (0, 0, 0)
 
@@ -307,6 +330,9 @@ class Motor(object):
     """
     Generic bi-directional motor.
     """
+
+    __slots__ = ('_forward', '_backward')
+
     def __init__(self, forward=None, back=None):
         if not all([forward, back]):
             raise GPIODeviceError('forward and back pins must be provided')