Update Animation.js

Added callback paramater to Animation.play( startTime, weight, callback );

I found this useful for chaining animations.

Example: 

var self = this; // object containing SkinnedMesh / animations / etc.

this.animations['Attack'].play(0,1,function(){

    self.animations['Idle'].play(0,1);

});

Perhaps there is an already existing better solution for this, but... perhaps not.