Move logic from `playerItem` change observer to setter

Instead of registering the `PRXPlayer` as an observer to itself, this
moves the logic that was in the `playerItem` observer to the
`setPlayerItem` method. In order to still be able to forward change
notifications to the delegate, a "fake" change dictionary is created and
passed along.
Merge pull request #10 from jballanc/master

Move logic from `playerItem` change observer to setter