Support 'S' elements with negative 'r' attributes in SegmentTimeline.

[tdrews]

A negative 'r' attribute indicates that a specified segment spans until the 't' attribute of the specified segment following it or to the end of the Period.

[priyajeet]

Thanks for getting this implemented. One thing I did notice was that the MPD fetching bypasses the request callback. Wouldn't requesting the MPD also go through the callback for consistancy?

[tdrews]

I assume that the above comment pertains to #148. Please see my response there.