Timing object spec

Hi.

Have reviewed section 1 through 4.

most notable change - added a section in the introduction concerning the
timing model implied by the timing object and how the timing object
connects with media element, text tracks and timing providers.

I felt these topics needed some introduction before we get to examples in
section 4, This section also provides some overview of the scope of the
document.

also added a paragraph on naming. Much less important though.

Ingar