Support restart and restartDefault animation attributes.

The 'restart' attribute can be used to prevent an animation from beginning if it has previously begun (restart never) or it is currently running (restart whenNotActive)

The (possibly inherited) 'restartDefault' attribute is used when the 'restart' attribute is not supplied.

http://www.w3.org/TR/smil/smil-timing.html#adef-restart
http://www.w3.org/TR/smil/smil-timing.html#adef-restartDefault
Merge pull request #87 from ewilligers/restartDefault

Support restart and restartDefault animation attributes.