Fix for BLD-708: Firefox YouTube video start time.

**Description**

In Firefox YouTube video with start time plays from 00:00:00. The reason was that seekTo() could not start playing a video from a point that hasn't been buffered yet.

**Reproducing the bug**
Use Firefox to reproduce this bug.

1.) Create a new unit in Studio.
2.) Add two videos. One should have Video URL set to http://youtu.be/OEoXaMPEzfM and the other should have Video URL set to http://clips.vorwaerts-gmbh.de/big_buck_bunny.webm .
3.) For each video, set the start time to 00:00:10, and the end time to 00:00:30 .
4.) Try playing both of them. Observe that the YouTube video starts playing from 00:00:00, even though the start-end range is displayed correctly. Observe that the second video starts playing from 00:00:10 (i.e. the expected, correct behavior).

**JIRA Issue**
[BLD-708](https://edx-wiki.atlassian.net/browse/BLD-708).

**Reviewers**
- @polesye 

**NOTES**
When [BLD-659](https://edx-wiki.atlassian.net/browse/BLD-659) will be implemented, many of the hacks (such as the `skipOnEndedStartEndReset` flag) which deal with start-end time will be refactored out.