Ensure observers are unregistered from playerItem
Ensure observers are unregistered from playerItem

This removes the KVO observer PRXPlayer instance from the player's
`playerItem` before it is deallocated. Note that this fix is only a
partial remedy, since the timing of when the observers are registered
leaves a small window during which the `playerItem` may be deallocated
before any KVO registration occurs.

jballanc/PRXPlayer,PRX/PRXPlayer,pingguoilove/PRXPlayer