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_source_tools.rst

@@ -0,0 +1,49 @@
+============
+Source Tools
+============
+
+.. currentmodule:: gpiozero
+
+GPIO Zero includes several utility routines which are intended to be used with
+the :attr:`~SourceMixin.source` and :attr:`~ValuesMixin.values` attributes
+common to most devices in the library. Given that ``source`` and ``values``
+deal with infinite iterators, another excellent source of utilities is the
+:mod:`itertools` module in the standard library.
+
+Single source conversions
+=========================
+
+.. autofunction:: negated
+
+.. autofunction:: inverted
+
+.. autofunction:: scaled
+
+.. autofunction:: clamped
+
+.. autofunction:: post_delayed
+
+.. autofunction:: pre_delayed
+
+.. autofunction:: quantized
+
+.. autofunction:: queued
+
+Combining sources
+=================
+
+.. autofunction:: conjunction
+
+.. autofunction:: disjunction
+
+.. autofunction:: averaged
+
+Artifical sources
+=================
+
+.. autofunction:: random_values
+
+.. autofunction:: sin_values
+
+.. autofunction:: cos_values
+