Bug fix: video end time proper seek beyond.

<b>Title</b>
"clicking outside of start/end time sends user to beginning of the video, instead of where she clicked."
https://edx-wiki.atlassian.net/browse/BLD-392 

<b>Description</b>
A related issue (bug?) is that when the user clicks on the video timeline either before or after the specified end time, it jumps to the very beginning of the video (even if it's before the specified start time).

<b>Technical details</b>
This seems to be a YouTube feature. When start/end time is specified, the video doesn't play beyond the end time, and restarts playing (upon finish) from the beginning.

Tested with vanilla Google code example + start/end time parameters at http://jsfiddle.net/4hVWv/ .

<b>Solution</b>
Do not use YouTube API `start` and `end` properties. Handle these cases manually. I.e. pausing the video when the time reaches `end` time.

<b>Reviewers</b>
- @singingwolfboy