Add support for @document

Adds support for @document rules per http://www.w3.org/TR/2011/WD-css3-conditional-20110901/#at-document. This has actually been removed from [the latest version](http://www.w3.org/TR/css3-conditional/), but is widespread in the data set I'm working with.

-Generalized the use of the media rules stack to cover all conditional rules. They all work the same (and can be nested inside each other), as far as I can tell.
-Though the spec says that the domain and url-prefix rules need quotes, the only supporting implementation so far (Mozilla) does not require them, so I've made them optional