Added a playback history object to the core.
TrackHistory stores Ref instances. Timestamp as epoch in milliseconds also stored.
Switched track name and artist order in history object.
Use assertIn instead of assertTrue to test membership.
history: Rename class to HistoryController

For consistency with other core controllers.
playback: Move comment so its next to the line it applies to
history: Use time.time() to get time since epoch
history: Import entire modules

For concistency with other code.
history: Keep history in private attribute

So it is not accessible directly from other actors.
history: Raise TypeError if non-Tracks are added
history: Cleanup docstrings
history: Tweaking formatting and variable names
history: Cleanup history entry test
history: Change size property to get_length() method

For consistency with tracklist.get_length() and our goal of aligning Python
and JS APIs by using less properties in the core API.
docs: Add HistoryController to core API
docs: Update changelog
http: Add history controller to JSON-RPC API
Merge remote-tracking branch 'mopidy/develop' into feature/history

Conflicts:
	docs/changelog.rst
Merge pull request #864 from jodal/feature/history

Add history controller to core API