Adding ability to compare previous/current states from a custom MyFollower class

Hey Kyle,

I'm new to your lib so excuse me if I didn't do this right…I followed your instructions at the top of Tracker.h to create my own custom MyFollower class.  My application requires that I kill off followers that don't meet a minimum velocity.  After a lot of trial and error, the only way I could figure out how to do was by storing the previous instance of the follower as a protected member of Follower and then having it update whenever tracker.track() is called.

If there's an easier/better way to do this then obviously please ignore (though I would be curious to know), otherwise maybe this could be of use to others.

Thanks for the lib!
Eric