Add pseudo selectors to kss.js/coffee

After accepting #52, I thought that perhaps we should support other types of pseudo selectors. 

This patch adds all other common pseudo selectors (with the exception of `nth-*` selectors).

I decided to PR this just to see if anyone else had a clever (read: clean) way to implement the `nth-child`, `nth-of-type`, and `nth-last-child` selectors.  

ping @kneath