Add position: sticke feature

Safari version is taken from [MDN](https://developer.mozilla.org/en-US/docs/Web/CSS/position#Gecko_notes).

Chrome version is taken from [this blog post](http://theamazingweb.net/2013/09/19/postion-webkit-sticky-test/). Chrome supports `sticky` from 23, but it requires change config flag.

Firefox from 26 version supports `sticky` but it still require config flag, so I mark it as unsupported.