adapt fragment loading/parsing logic to support progressive buffering

not implemented yet, but should be easier after this refactoring.

provide 3 callback functions to demuxer :
function _fragParsingAudioSelectionHandler(audioTrackList : Vector.<HLSAudioTrack>) : HLSAudioTrack
demuxer provides to fragment Loader the list of audio tracks found. fragment loader will then tell demuxer which one needs to be selected

function _fragParsingProgressHandler(audioTags : Vector.<Tag>, videoTags : Vector.<Tag>) : void
callback for progressive parsing

function _fragParsingCompleteHandler(audioTags : Vector.<Tag>, videoTags : Vector.<Tag>) : void
callback for parsing complete

related to https://github.com/mangui/HLSprovider/issues/140