Integrated Transcript

Major changes to add an automatic integrated transcript, including addition of a hopefully/mostly compliant WebVTT parser.

Also changed current caption/description displays to use data from WebVTT parser (rather than doing their own ad-hoc parsing.)

Still to-do:
* Determine exactly how the transcript should be laid out on the page.  Right now, it is placed below the player (since vertical additions are easier than horizontal.)  Do we want the developer to pass in a div to place the transcript in?  Do we want the user to be able to turn the transcript on/off?  How would these two features interact?

* (Bug) Play button changes to pause even when player does not begin playing, after clicking on transcript text.

* Several places in the code were referring to hasTranscript during initialization; if users are going to have control over transcript display, these places will need to be dynamically updated instead.  Removed usages of this where it seemed appropriate; might want to change preferences page?