Add support for animation-fill-mode

As far as I can tell not all properties are covered in the tests. I personally find it useful to have a regression test for such a case. So I added one although I'm not sure if I placed it in the right file.

According to http://www.w3schools.com/cssref/css3_pr_animation-fill-mode.asp only webkit needs a prefixed version of `animation-fill-mode`.

Could you please review the code. Thanks. :smiley: