Fix #140, fix #69, fix #185

This PR adds a software SPI implementation. Firstly this removes the
absolute necessity for spidev (#140), which also means when it's not
present things still work (effectively fixes #185), and also enables any
four pins to be used for SPI devices (which don't require the hardware
implementation).

The software implementation is simplistic but still supports clock
polarity and phase, select-high, and variable bits per word. However it
doesn't allow precise speeds to be implemented because it just wibbles
the clock as fast as it can (which being pure Python isn't actually that
fast).

Finally, because this PR involves creating a framework for "shared"
devices (like SPI devices with multiple channels), it made sense to bung
Energenie (#69) in as wells as this is a really simple shared device.

gpiozero/output_devices.py

@@ -10,8 +10,9 @@ from time import sleep
 from threading import Lock
 from itertools import repeat, cycle, chain
 
-from .exc import OutputDeviceBadValue, GPIOPinMissing, GPIODeviceClosed
-from .devices import GPIODevice, GPIOThread, CompositeDevice, SourceMixin
+from .exc import OutputDeviceBadValue, GPIOPinMissing
+from .devices import GPIODevice, CompositeDevice, SourceMixin
+from .threads import GPIOThread
 
 
 class OutputDevice(SourceMixin, GPIODevice):
@@ -38,10 +39,8 @@ class OutputDevice(SourceMixin, GPIODevice):
         device will be switched on initially.
     """
     def __init__(self, pin=None, active_high=True, initial_value=False):
-        self._active_high = active_high
         super(OutputDevice, self).__init__(pin)
-        self._active_state = True if active_high else False
-        self._inactive_state = False if active_high else True
+        self.active_high = active_high
         if initial_value is None:
             self.pin.function = 'output'
         elif initial_value:
@@ -72,6 +71,10 @@ class OutputDevice(SourceMixin, GPIODevice):
 
     @property
     def value(self):
+        """
+        Returns ``True`` if the device is currently active and ``False``
+        otherwise. Setting this property changes the state of the device.
+        """
         return super(OutputDevice, self).value
 
     @value.setter
@@ -80,7 +83,22 @@ class OutputDevice(SourceMixin, GPIODevice):
 
     @property
     def active_high(self):
-        return self._active_high
+        """
+        When ``True``, the :attr:`value` property is ``True`` when the device's
+        :attr:`pin` is high. When ``False`` the :attr:`value` property is
+        ``True`` when the device's pin is low (i.e. the value is inverted).
+
+        This property can be set after construction; be warned that changing it
+        will invert :attr:`value` (i.e. changing this property doesn't change
+        the device's pin state - it just changes how that state is
+        interpreted).
+        """
+        return self._active_state
+
+    @active_high.setter
+    def active_high(self, value):
+        self._active_state = True if value else False
+        self._inactive_state = False if value else True
 
     def __repr__(self):
         try: