handle NAL Unit spread accross continuous  segments
reuse demux instance when loading continuous segments of same level, this allows to reassemble PES/NAL Unit correctly.
add a flush method in Demuxer.as interface, useful to flush remaining data at end of stream / before discontinuity
remove all static variable from TSDemuxer.as
related to https://github.com/mangui/flashls/issues/31
related to https://github.com/mangui/flashls/issues/7