Revision timing object spec - section 10 on timed data

Hi all.

I've revised section 10 concerning how the timing object may be used to
drive timed presentation of timed data.

- Added an introduction
- removed changes to the timing object definition in the interest of
keeping a clear separation between timing object and the timing text track
- defined two approaches
1) library approach : generic JavaScript sequencer
2) integration with existing standards: timed text tracks
- added some issues for discussion

I'm still a little worried that the introduction of timed text tracks might
not be extremely valuable (see issues) and I would like to have your
feedback on this issue - or questions if the issue is not clear.

Best,

Ingar