Update Player timing model and API to match spec

http://dev.w3.org/fxtf/web-animations/#players
http://dev.w3.org/fxtf/web-animations/#the-player-interface

Player currentTime is now bounded by the source content unless seeked beyond the boundary.
Players can become held to a certain currentTime value.
The Player API has been updated to reflect the new timing behaviour.

This change depends on #536 landing first.