avoid reporting buffer progress event too often
split out dynamic level switch from FragmentLoader
move it to AutoLevelManager.as
fix https://github.com/mangui/HLSprovider/issues/46
remove dependencies on global levels Array (make the code re-entrant)
improve dynamic switching algorithm, especially level switch down, which should be more aggressive now.
it was not following the spec (http://www.cs.tut.fi/~moncef/publications/rate-adaptation-IC-2011.pdf)

fix https://github.com/mangui/HLSprovider/issues/47