Support @document conditional rules

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, but is widespread in the data set I'm working with.

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

@document is a specific case of conditional rules. Another example is @media. I reworked and generalized the existing media rule support. Specifically, CSS:Media is now for entire @media rules and not just the media names ("screen", "print", etc.). Media and document rules should be nestable inside each other.