Update tests to catch scaling problem in mixer.

- Fixes problem where 60% became 59% due to bad rounding.
- Tests assume scale of 0-65536 which matches ALSA.
- Check all possible values of set_volume and ensure we the right value out.
Removed autoaudiosink's device selection.

The way this code was testing devices locked the element to using the wrong
device. The incorrect device had a max volume of 39 on the Master track, really
making accurate volume changes impossible.

Instead of trying to make any guesses about this I'm leaving it to the element
to have sensible defaults.

Code will also ensure that it returns a newly created copy of the mixer, not
one we have already used.
Turn autoaudiomixer into a bin.

This allows us to add our sub mixer that we are proxing (not sure if
GstChildProxy can be used in Python) so that state changes to the parent
propagates nicely.
Update code to handle case where AutoAudioMixer fails to find a mixer.

This change implies removing the GstMixer interface from the autoaudiomixer,
which allows us to check if a mixer was found in a generic way by using
get_by_interface. This also means we get direct access to the child mixer so
the proxying code is no longer needed.
Cleanup error handling of bad GStreamer settings.

No need to have a GStreamerError, we can just let the GErrors bubble now that
we are initialising in the main thread.
Remove MIXER_MAX_VOLUME setting.
Fix MPD volume command.

The command should return -1 when the volume is not known.
Reraise exception without losing the traceback (fixes #173)
Merge remote-tracking branch 'adamcik/feature/simplify-outputs' into develop

Conflicts:
	docs/changes.rst
	mopidy/utils/settings.py
Tweak OUTPUT docs
Improve docstring
Map old settings GSTREAMER_AUDIO_SINK and LOCAL_OUTPUT_OVERRIDE to the new OUTPUT setting
Use same wording in setting validation and docs
Cleanup error handling of bad GStreamer settings.

No need to have a GStreamerError, we can just let the GErrors bubble now that
we are initialising in the main thread.

Conflicts:

	mopidy/gstreamer.py
Fix merge bummer
Change boolean arg to kwarg to make it selfdocumenting
Merge branch 'develop' into feature/switch-to-gst-mixers

Conflicts:
	mopidy/gstreamer.py
Merge branch 'develop' into feature/switch-to-gst-mixers

Conflicts:
	mopidy/gstreamer.py
Add issues that was fixed by output simplification to the changelog
Another issue fixed by simplified outputs