Remove unused observer

The `mediaPlayerDidChange:` method is removed since it was only being
called by the `PRXPlayerAVPlayerContext` notification which is no longer
registered on any key path. The functionality in `mediaPlayerDidChange:`
had previously been moved to the `setPlayer:` method.
Refactor item change notification to fix observers

In order to keep proper accounting of observers added and removed from
the `AVPlayerItem` in the current player, the
`mediaPlayerCurrentItemDidChange:` method that is called on a player
item change now utilizes the change dictionary to get the old and new
items and register and deregister notifications appropriately. In order
for this to also work on player change notifications, a dummy change
dictionary is constructed and passed along.
Merge pull request #9 from jballanc/playerItem-kvo-refactor

Refactor playerItem KVO registration