The source/values toolkit

Me and my big mouth. No sooner do I declare the base classes "relatively
stable" than I go and mess around with it all again. Anyway, this is the
long promised set of utilities to make source/values more interesting.
It includes a few interesting little utility functions, a whole bunch of
examples and introduces the notion of "pseudo" devices with no (obvious)
hardware representation like a time-of-day device.

This necessitated making the event system a little more generic (it's
not exclusive the GPIO devices after all; no reason we can't use it on
composite devices in future) and by this point the mixins have gotten
large enough to justify their own module.

The pseudo-devices are a bit spartan and basic at the moment but I'm
sure there'll be plenty of future ideas...

docs/api_other.rst

@@ -0,0 +1,14 @@
+================
+Internal Devices
+================
+
+.. currentmodule:: gpiozero
+
+GPIO Zero also provides several "internal" devices which represent facilities
+provided by the operating system itself. These can be used to react to things
+like the time of day, or whether a server is available on the network.
+
+
+.. autoclass:: TimeOfDay
+
+.. autoclass:: PingServer