Implemented the other end of the webvtt timestamp parsing as it will
be needed when we write out WebVtt files. Also changed the name of
the parse function to be more inline with the writing function.
Change-Id: I1f36ddbbf80028732b4cb1b15e871dec17767f63
Moved all the timestamp parsing code into its own class with its
own unit tests. This is to make testing easier.
Change-Id: I624472baba51dfa1254a9f4bae55ebb79e310855