Fixed exception when using Song class before MediaManager is initialised.
Added shutdown of media manager when game exits.
Update MediaManagerState.cs

Removed thread safety checks from media manager, as it should only be started or stopped from the UI thread anyway.

Caller should be responsible for ensuring this.
Merge branch 'develop' of https://github.com/SimonPStevens/MonoGame into SimonPStevens-develop

Conflicts:
	MonoGame.Framework/Media/MediaPlayer.cs
Merge pull request #1820 from totallyeviljake/PR_1509_UPDATE

#1509 Updated to resolve conflict - verified by @borrillis