Wrap vtt.js in a function expression.

Also export WebVTTParser on the global whether we
are running in node or otherwise. This was suggested
by Dave on bug 895091 to take care of the node,
web page, and browser use cases.

Done in response to comment https://bugzilla.mozilla.org/show_bug.cgi?id=895091#c93 by @humpd.

I've tested on Gecko locally and it works perfectly. It should work fine on B2G as well.