Added an unit test to the linear curve function used by animations.
Added the acceleration function to the table vega.animations.curves.
Added the slowdown function to the table vega.animations.curves.
Added the accelerationandslowdown function to the table vega.animations.curves.
Added the slowdownandacceleration function to the table vega.animations.curves.
New vega.animations.hide function to create an animation that hides a drawable.
New vega.animations.show function to create an animation that change the visibility of a hidden drawable.
New vega.animationtrack function.
Deleted an unused file.