Added easing functions

Hi mhuggins,

I really like your plugin, nice and simple and does what I needed it to. A colleague of mine wondered if I could add easing to the counting so I forked your plugin and added the necessary code today.

You can see it in action on the "Earth's Gravity" and "Earth's Radius" examples.

Unfortunately I haven't had time to update the readme.

If you like this please feel free to merge in these changes. Enjoy!

Steve