Added notes on how the abstracts are represented, ensured all the class
hierarchies were up to date, and changed the orientation so the classes
are actually readable in the big chart.
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...
