Add .stuck class

[euancm]

Hey,
I thought it would be helpful to add a .stuck class to make it possible to style stuck elements.

I've never really used github before, but seeing as I got it working with only one line, I don't think its worth doing a pull/fork/push whatever its supposed to be.

Anyway, after line 38 in polyfill.position-sticky.js, add

    $(data.$clone).addClass('stuck')

then you can style the element with .element.stuck {

It's not a perfect solution due to the way the polyfill works, but it works for small effects.

Thanks,
euan.