Converted _tweens from an Array to an Object

Hey @sole,

Happy New Year! I've done a much better job following your instructions :) This breaks the API a bit ( I had to rewrite all the tests ), because you can't just call `TWEEN.getAll().length` you have to call `TWEEN.getLength()`, but I've noticed it's a lot faster because it's easier to access, add, and remove tweens. As I made my findings I figured I'd share with you via Pull Request.

Cheers.