fixes switching the playback  to 'stalled' state if any of buffers is empty
report invalid mpd
when stalling a buffer make sure that segments are scheduled so that it can be resumed eventually.
makes clickable only a non-transparent part of "Fork me on Github" ribbon
resolves a pending deferredInitAppend promise before it is reinitialized instead of rejecting it
fixes availabilityWindow calculations for SegmentTimeline
fix issue #131