Switch to rust-url

Fixes #57.

I’m making a branch of Servo based on this. I’m not sure if you want it, but here it is.

This adds a dependency on https://github.com/SimonSapin/rust-url , which itself depends on https://github.com/lifthrasiir/rust-encoding. The appropriate `-L` flags need to be passed in `RUSTFLAGS`.

rust-url itself uses Cargo, but I’ve made a Makefile for Servo. (Servo wants cross-compiling, which is not supported by Cargo yet.) https://github.com/SimonSapin/servo/tree/rust-url/src/support/url
Admittedly this is gratuitous and not necessary for the purpose of this PR. IMO the strongly-typed header system in rust-http is broken. It does not preserve the original bytes, and causes task failure when a value is not formatted as expected. This is a small step towards reducing the damage. I’m happy to discuss API design more, but maybe this PR is not the place to do so.