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.
Start-end time range always shown, even before video plays.
Added Jasmine test for initial retrieval of duration.
Updated Changelog.rst file.
Addressing singingwolfboy's comments from 2014-01-15 11:45:03, and 11:46:59.
Addressing polesye's comments from 2014-01-16.
Addressing polesye's comments from 2014-01-17.
Addressing polesye's comments from 2014-01-17 [part 2].

Moving of initialize() and initializeYouTube() functions
to helper.js file.
Addressing polesye's comments from 2014-01-17 [part 3].

More elegant handling of multiple metadata AJAX requests.