Dispatch an event when subtitles are updated in subtitle div.

These 3 commits (2 code commits) resolve issues #8 and #9 by:
* Reducing the draw calls to the DOM by only changing the contents of the subtitle div when the text of the subtitle should change (rather than every tick)
* Dispatching a 'subtitlechanged' event to the DOM with information about the `<video>` tag, timecode and subtitle.