This removes the circular dependency introduced in PR#137. This also
fixes up an issue in the base meta-class which meant it wasn't working
in Python 3 (only Python 2), and adds a bit to the meta-class to allow
docstrings to be inherited (taken from the rest-docs branch).
Add forward_device and backward_device to Motor, left_motor and
right_motor to Robot, and ensure all CompositeDevice descendents have a
proper close() method and closed property. Also, add a few more
_check_open calls around the place to make sure GPIODeviceClosed is
properly raised in response to read and writing values.
This finishes off implementing values and source for all (current)
classes in gpiozero. I'm afraid things get rather complex in this
commit. For starters, we've now got quite a few "aggregate" classes
which necessarily don't descend from GPIODevice. To implement values and
source on these I could either repeat a helluva lot of code or ... turn
to mixin classes. Yeah, it's multiple inheritance time, baby!
Unfortunately multiple inheritance doesn't work with __slots__ but we
really ought to keep functionality that they provide us (raise
AttributeError when an unknown attribute is set). So I've implemented
this with ... erm ... metaclasses. Sorry!
Big push on getting the docs cleaned up before 1.0. Proper wrapping of
everything so it's decently viewable from the command line (or as
decently viewable as markdown can be - the tables will never look great
from the command line).
Only one code change in this PR: rename bouncetime to bounce_time
(everything else is PEP-8, so this probably should be too) and change
its units to seconds from milliseconds (again, all other durations in
the library are in seconds, so it feels inconsistent that this one
isn't; for the sake of those who won't read the docs - which is most
people - I figure consistency helps with guessing!).
