Show start time or starting position on slider and VCR.

When the start time is set, or a initial position is
set,the VCR timer and the time slider should jump to
correct starting point before the video starts playing.

BLD-823
Merge pull request #2744 from edx/valera/move_slider_and_vcr_to_start_time_2

Show start time or starting position on slider and VCR