Rebuffering notification and CocoaPods

We needed audjustable to inform the delegate when the buffers dry up. We added a new internalState (AudioPlayerInternalStateRebuffering) which should have no side effects.

We needed to have Audjustable in Cocoapods so we added our fork (http://cocoapods.org/?q=audjustable). Let us know if you want to take that over.

We also reverted AudioPlayerBuffersNeededToStart back to 16, I'll see about adding this as a parameter to either the constructor or setDataSource in a future pull request.