Rename Player to AnimationPlayer

Renamed discussed in Web Animations minutes 28 Feb 2014
https://etherpad.mozilla.org/ep/pad/view/ro.WCBLOuse6D5/latest

Web Animation spec, Section 5.2 The AnimationPlayer interface
http://dev.w3.org/fxtf/web-animations/#the-animationplayer-interface
Merge pull request #571 from ewilligers/AnimationPlayer

Rename Player to AnimationPlayer