TODO

- docs
- paths
    * bezier
- easing
    * stepped ease for retro shaky animations    
-proxy        
    * callback proxy
- timeline control
    * ffw/rewind/seek/elasetic time factor
    * sequence/parallel sugar
    * delay before or after
- tween types
    * more color transforms
    * sprite animation
    * spawner
    * rotation/scaling
    * distinct int optimization in 2D for position, and for array proxy
- demos
    * add trails to easing demo, make them tween fade
    * rain falling
    * shape tweening using lots of particles
    * theatrical demo showing parallel and sequenced tweens
- API features
    * validation should be in validation classes to get rid of all those ifs in constructor
    * lifecycle events -> start,stop,complete
    * speed instead of duration
    * better safety and error messages on tween misconfiguring
    * tail behavior event on tween target going out of scope
- performance
    * share path_args for polylines
    * pool tweens
    * convert doubles to floats
- accuracy
    * overshoot support