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.

gpiozero/__init__.py

@@ -121,19 +121,3 @@ from .other_devices import (
     PingServer,
     TimeOfDay,
 )
-from .source_tools import (
-    averaged,
-    clamped,
-    conjunction,
-    cos_values,
-    disjunction,
-    inverted,
-    negated,
-    post_delayed,
-    pre_delayed,
-    quantized,
-    queued,
-    random_values,
-    scaled,
-    sin_values,
-)