TTML Level 1 API

I've drafted a basic "level 1" API for TTML at [1]. I expect to draft a
"level 2" version shortly, which builds upon (but doesn't replace) this
level 1 API.

My intent in dividing the API into two levels is that level 1 should
support the exposure of unparsed TTML1 (or TTML2) content to client script
for disposition by client script (only). And that level 2 should support
exposure of parsed/renderable TTML1 (or TTML2) content, using the HTML/CSS
mapping to be defined in TTML2 (or perhaps in a separate spec that could be
applied to TTML1 content as well as TTML2).

[1] https://dvcs.w3.org/hg/ttml/raw-file/default/ttml1-api/Overview.html