Fix #25

Add a nice __repr__ to the GPIODevice base class.

This isn't much but generally I think `__repr__` implementations should
be deliberately simple: firstly, they're frequently used for debugging
so if they're at all complex you risk making a debugging tool buggy
(very annoying!). Secondly, if you pour too much info into them you risk
making the debugging output cluttered, so I tend to prefer keeping it to
straight-forward simple to retrieve/calculate info without excessive
detail (if the user wants more, they can always query it directly).

There is one refinement here: in SmoothedInputDevice, `__repr__` is
tweaked to ensure that when partial is False (the default), and the
queue isn't filled, `__repr__` doesn't block (because it should *never*
block).

gpiozero/devices.py

@@ -31,6 +31,10 @@ class GPIODevice(object):
     def is_active(self):
         return self._read()
 
+    def __repr__(self):
+        return "<gpiozero.%s object on pin=%d, is_active=%s>" % (
+            self.__class__.__name__, self.pin, self.is_active)
+
 
 _GPIO_THREADS = set()