Fix start-time functionality in the video player.

When there is a start-time:

Initially, the video starts playing from the start-time.
Whenever the player resets for any reason (such as reaching
the end of the video), it will reset to start-time instead of 00:00:00.
It should not be the case that any time the user presses play, it
start playing from start-time.

BLD-659.
Addressing review comments from jmclaus.
Addressing review comments from jmclaus.
Merge pull request #2690 from edx/valera/add_start_time_end_time_functionality

Fix start-time functionality in the video player.