Start at adding media events for audio/video elements. Note, the underlying implementation for HTML5 media
event handling is expected to change in the near future.

Review at http://gwt-code-reviews.appspot.com/1385804

Review by: pdr@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9887 8db76d5a-ed1c-0410-87a9-c151d255dfc7