Make vtt.js usable in a browser, add WebVTTParser.StringDecoder for string data

This makes it easier to use vtt.js in a browser, including:

* `grunt build` to create `dist/vtt.min.js` with vtt.js and the encodings.js polyfill
* adds support for simple string parsing via a new StringDecoder
* adds another test type for parsing all vtt files as strings within `jsonEqual`
* fixes cases where string parsing is impossible by adding `jsonEqualUTF8` to force binary parsing
* updates README with info on the above