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...

gpiozero/boards.py

@@ -22,7 +22,8 @@ from .exc import (
 from .input_devices import Button
 from .output_devices import OutputDevice, LED, PWMLED, Buzzer, Motor
 from .threads import GPIOThread
-from .devices import Device, CompositeDevice, SharedMixin, SourceMixin
+from .devices import Device, CompositeDevice
+from .mixins import SharedMixin, SourceMixin
 
 
 class CompositeOutputDevice(SourceMixin, CompositeDevice):