Migrate from liburl to rust-url

The standard url library is soon-to-be deprecated, and now that we're
bootstrapping it's trivial to move over to rust-url.

Closes #204
auto merge of #297 : alexcrichton/cargo/url, r=wycats

The standard url library is soon-to-be deprecated, and now that we're
bootstrapping it's trivial to move over to rust-url.

Closes #204