Add public/private marks to the animator implementation.

[jverkoey]

This will help organize some upcoming work on the animator's implementation.

[samnm]

I usually put private stuff at the bottom of the file, but I don't know if either is preferred.

[romoore]

+1 to private at the bottom. I'm assuming all the methods that follow, including animateWithTiming:toLayer:withValues: is private as well?
I just saw the #public afterward. I think it's clearest to put the private at the bottom. Then anything no explicitly marked (-init?) can be assumed public.