Add a submodule for rust-cssparser

Even though this parser is not used yet, having it built
and tested with the rest of Servo helps ensure it does not bitrot.
auto merge of #568 : SimonSapin/servo/rust-cssparser-in-tree, r=metajack

[As discussed](https://github.com/mozilla/servo/wiki/Meeting-2013-07-01#css), even though this parser is not used yet, having it built and tested with the rest of Servo helps ensure it does not bitrot.