Animation constructor easing

Fix bug https://github.com/web-animations/web-animations-next/issues/131.

Timing functions weren't working with explicitly constructed Animations because normalizeTimingInput was being called twice.

Now we just store the raw timing input in the Animation object, and call normalizeTimingInput any time we need a normalized timing object.

@shans ptal
I don't think this is correct.
This means that Animation(null, null, 1234).timing will return 1234 when it should return an AnimationTiming object.
http://dev.w3.org/fxtf/web-animations/#idl-def-AnimationTiming