Movement command, Animation State

Addresses #6 
Everything works as far as I can tell, but I expect the style will need tons of fixing, since I'm new to javascript. I don't mind making lots of changes, so go ahead and tear this apart.
In addition to the MovementCommand and AnimationState classes, It made sense to me to make a Coordinate struct that represents coordinate pairs ({x: __, y: __}). Of course, we didn't specifically talk about that in the original plans, so I can remove it if you don't like it. 
It's true that javascript only has a Number type, but usually the best practice is to distinguish between `int` and `float` in the documentation instead of simply `Number`.