Update Player to match new spec behaviour and API

This change brings the polyfill up to speed with the spec with regard to Players.
http://dev.w3.org/fxtf/web-animations/#players
http://dev.w3.org/fxtf/web-animations/#the-player-interface

The changes involved are:
 - Making the currentTime of a player bounded to the duration of its source content.
 - Adding the following to the Player API: playing, cancel(), finish(), play(), pause(), reverse().
 - Making the paused attribute on Players readonly.
 - Updating tests to the new behaviour.