improve seeking & remove Video object from HLS

#### ChangeLog

- remove video from HLS and Buffer classes; add NetStream instead. One of the reasons: we can use audio only streams and don't need Video at all;

- add _accurate seeking_. Before position after seek was equal to current packet's start position. But we can seek more precisely by not appending some first tags. See ```_filterTag(tag:Tag,pts:Number)```. Use ```HLS.seekAccurate(flag:Boolean)``` to turn on/off this feature. Turned off by default;

- some state switching fixes: seek in "paused" state should not switch to "playing" after seek complete; we don't need "buffering" state right before "idle";


P.S. Sorry, my IDE broke indentation. Too many changes in _diff_