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/devices.py

@@ -107,7 +107,13 @@ class GPIOMeta(type):
                         try:
                             old_close()
                         finally:
-                            del cls._INSTANCES[key]
+                            try:
+                                del cls._INSTANCES[key]
+                            except KeyError:
+                                # If the _refs go negative (too many closes)
+                                # just ignore the resulting KeyError here -
+                                # it's already gone
+                                pass
                 self.close = close
                 cls._INSTANCES[key] = weakref.proxy(self)
         else: