Make tests work reliably on the Pi

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.

docs/api_tools.rst

@@ -0,0 +1,56 @@
+============
+Source Tools
+============
+
+.. currentmodule:: gpiozero.tools
+
+GPIO Zero includes several utility routines which are intended to be used with
+the :attr:`~gpiozero.SourceMixin.source` and
+:attr:`~gpiozero.ValuesMixin.values` attributes common to most devices in the
+library. These utility routines are in the ``tools`` module of GPIO Zero and
+are typically imported as follows::
+
+    from gpiozero.tools import scaled, negated, conjunction
+
+Given that :attr:`~gpiozero.SourceMixin.source` and
+:attr:`~gpiozero.ValuesMixin.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
+