Add Servo and AngularServo implementation along with docs and tests.
This is a deliberately minimal implementation designed to be added to as
we agree on new extensions (better than making an all-singing,
all-dancing version in which I get things wrong and then wind up making
backward incompatible changes to get it right :)
While the tests work well on a PC or Travis, the Pi (where I ought to be
running them!) has some issues with the timing tests. Need to relax the
tolerance of the "assert_states_and_times" method to 0.05 seconds
otherwise it periodically fails even on something reasonably quick like
a Pi 2 (less failures on a Pi 3 but still occasionally).
Also reduced default fps to 25; if the default timing occasionally fails
on a Pi 2 it's evidently too fast for a Pi 1 and shouldn't be the
default; 25 also doesn't look any different to me on a pulsing LED.
There's also a bunch of miscellaneous fixes in here; last minute typos
and chart re-gens for the 1.2 release.
Change MockPin (and MockPWMPin) to make them behave more like 'real' pins - fixes#206
Add new MockPin tests, and rework some of the existing ones
Incorporate #216
Doing timing tests on pypy (for blink) just doesn't work; even with a
tolerance as coarse as tens-of-milliseconds, things fail. I guess that
isn't too surprising given that, as the tests are one off things the
pypy JIT is never getting a chance to spin up and do its stuff.
