Fix repeated pausing of Player
Change the behaviour of setting Player.currentTime and startTime

- Setting currentTime seeks by updating the drift. It does not affect the
  startTime.
- Setting startTime seeks by updating the internal startTime and hence the
  currentTime. It does not affect the drift.
Make sure the polyfill is running after seeking Player
Merge pull request #104 from steveblock/seekPlayer

Seek player
Update transform tests to use Player
Merge pull request #103 from steveblock/repeatedPause

Fix repeated pausing of Player
Merge pull request #105 from steveblock/transformTests

Update transform tests to use Player
Make Timeline.currentTime a read-only property rather than a method

See https://github.com/web-animations/web-animations-js/issues/107
Use the document load event as the zero point for document time

Also make sure that the document time is null before this zero point, and add a
test to check that the document time is constant within a JavaScript
callstack.

This fixes https://github.com/web-animations/web-animations-js/issues/101