Allow scrollToDate() to take a callback

[GoogleCodeExporter]

Right now, there's no way to assign a callback to scrollToDate, or to stop the 
animation once it starts. 

- The function should take a callback parameter, to be fired at the end of the 
animation. (This requires some ugly monkey-patching of Timeline)

- It would be nice if the function could, if animated, return the animation 
object, which could include a stop() method. (I think this is built into 
Timeline)

Original issue reported on code.google.com by [email protected] on 6 Mar 2012 at 5:49