Update API to match spec.

* rename types
* add missing types
* generate errors when accessing internal constructors
Merge pull request #153 from dstockwell/spec-update

Update API to match spec
Add Timeline.getCurrentPlayers

From http://lists.w3.org/Archives/Public/public-fx/2013JanMar/0063.html
Merge pull request #154 from dstockwell/current-players

Add Timeline.getCurrentPlayers
Add Player.timeline
Fix calculation of Player.currentTime when Timeline is not started
Add Player.playbackRate

Also remove TimedItem.changePlaybackRate().

This fixes https://github.com/web-animations/web-animations-js/issues/135
Merge pull request #155 from steveblock/playbackRate

Implement Player.playbackRate and fix a couple of minor bugs
Remove test boilerplate
Move initialization into extra-asserts and rename anim-test-pre
Update testcases
Fix typo
Use animTestRunner in test-generator
Fix frame forward/back
Fix play
Merge pull request #156 from dstockwell/less-boilerplate

Reduce boilerplate in new test framework
Remove arguments from TimedItem._isCurrentDirectionForwards()
Clarify TimedItem._isPastEndOfActiveInterval()

No functional change
Initial implementation of media integration