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/exc.py

@@ -13,6 +13,9 @@ class GPIOZeroError(Exception):
 class DeviceClosed(GPIOZeroError):
     "Error raised when an operation is attempted on a closed device"
 
+class BadEventHandler(GPIOZeroError, ValueError):
+    "Error raised when an event handler with an incompatible prototype is specified"
+
 class CompositeDeviceError(GPIOZeroError):
     "Base class for errors specific to the CompositeDevice hierarchy"