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

- The tag name for cuetext class tags should be span according to the
WebVTT DOM construction rules.
- The local names of the generated HTML elements should be set to the name of the HTML tag rather then the cuetext tag.

Relevant spec rules [here](http://dev.w3.org/html5/webvtt/#webvtt-cue-text-dom-construction-rules).