added TweenChain

TweenChain component:
* sequentially arranges tweens (one tween plays after another)
* optionally loops
* can be called with relative offsets (think `{x: -2*TILE_WIDTH}` instead of `{x: ent.x - 2*TILE_WIDTH}`
* does not interfere with already present Tweens or future Tweens
* downside?: only one TweenChain at a time; possible to implement for more if needed

[Example JSFiddle](http://jsfiddle.net/yQ7NE/) showing 1 TweenChain and 2 other Tweens animating at same time

This feature is listed in the [Idea pool - Path Movement](https://github.com/craftyjs/Crafty/wiki/Idea-Pool) and should replace #515

Tests coming after I get the O.K. on the code