convert to more UI friendly buttons
adds initial TTML support
adds a verification that TTML document conforms to timing and structural constraints
fixes IndexSizeError when removing TextTrackCue's in TextTrackExtensions.deleteCues
resets textSourceBuffer when switching mpd's
ttml bugs fixes
handles ttml parsing errors
fixes structural constraints check
adds test for 'profile' element to TTMLParser.passStructuralConstraints
fixes jshint error