keep period around for use later
some bad changes to load segments

This is a bad way to allow the buffercontroller to load segments before
the video element currentTime is set. this should cooperate more with
the video element and the stream to explicitly load segments at a time
different from video.currentTime
fix some bugs in stream

- periodInfo must be set after data
- buffers must load segments for the start time before video.currentTime
is set
- use calcPresentationTime to get the start time
fixes for start and end time calculations.