Add JS doc comments.

I added documentation comments in the JSDoc style throughout the whole project. Most of the comments are some what of a placeholder for now, as my main focus was the public facing API (ReaderView).

Check out the auto generated documentation here:
https://dl.dropboxusercontent.com/u/18642964/Readium/Shared-JS%20JSDoc/index.html
Callback emitted from the app layer (i.e. native TTS engine) that notifies the JS player about the completion of the current synthetic speech utterance.