Update class cuetext tags to create span elements.

The tag name for cuetext class tags should be span according to the
WebVTT DOM construction rules.
Update elements to store the correct local name.

Due to this I had to introduce a tag stack that will keep track
of the stack of cuetext tags that have yet to be closed.
Merge pull request #88 from RickEyre/class-fix

Update element's local name and tag name to be to spec