Add videos for testing MediaReference

This will be published on a gh-pages branch to allow them to be served by
github. See
https://github.com/web-animations/web-animations-js/pull/316#issuecomment-20329158
Merge pull request #340 from steveblock/videosForTesting

Add videos for testing MediaReference
Fix MediaReference after switch to new Timing interface

This was broken by 5ee65b94527a1c0808d89f2ba229d88303310332 and following commits.

Also fix the test after the switch to the new testing framework.
Fix MediaReference to stop the media when detached from its Player