Enable correct total time display in VCR for YouTube videos.

Before this, the VCR showed total time as 00:00:00 for YouTube
videos just after page loaded. The correct total time is shown only
after the user clicks Play button.

BLD-529.
Start-end time range always shown, even before video plays.

BLD-529.
Added Jasmine test for initial retrieval of duration.

BLD-529.
Updated Changelog.rst file.

BLD-529.
Addressing comments for PR 2176.

BLD-529.
Merge pull request #2176 from edx/valera/fix_start-time-end-time_2

Valera/fix start time end time 2