Bug fix: video end time proper seek beyond.

This is a work in progress.

https://edx-wiki.atlassian.net/browse/BLD-392 "clicking outside of start/end time sends user to beginning of the video, instead of where she clicked."

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).