Start dogfooding rust-url

See https://github.com/SimonSapin/rust-url
The old liburl is [going away](https://github.com/rust-lang/rust/issues/10707#issuecomment-49107880) eventually.

This depends on https://github.com/mozilla-servo/rust-http/pull/23

This adds https://github.com/SimonSapin/rust-url.git as a submodule. We might want to move it into github.com/mozilla-servo.