Subtitle support

This is **not** ready to merge.

This pull request adds support for vtt, srt, and ass subtitles to MediaCrush.

![2014-02-06_22 54 18](https://f.cloud.github.com/assets/1310872/2107293/e40c3794-8fc4-11e3-8b68-5b75f93f1854.png)