Refactor animation.js to use a time based rather than frame based approach.

Crafty.easing
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- A new object `Crafty.easing`; it abstracts the concept of a state that changes over time.
- Used in the new "Tween" and "SpriteAnimation" components; can also be used when we fix the viewport animation functions.
- No public documentation right now, since it's for internal use.
- Added tests in core.html

Tween
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- Uses the new easing object
- Now treats a set of properties to tween as a group, only sending one "TweenEnd" event per group
- Removes some special handling for mouse events that I'm pretty sure is already handled by how the "2D" component works.
- Updated the docs and added tests in core.html

SpriteAnimation
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- Uses the new easing object
- Refactored the interface completely.  Uses `reel()` to set and change reels, `animate()` to play them.  Favors composable single purpose methods over tons of optional arguments.  See documentation/source/tests for full details.
- The duration of an animation is now set in milliseconds when creating a reel, instead of in frames when playing it.
- Updated the docs and tests in animation.html
Merge pull request #583 from starwed/animation-overhaul

Refactor animation.js to use a time based rather than frame based approach.