(feat) Embed a url::Url inside every Request.

I've been playing around with putting a proper `Url` object inside every `Request`.

Unfortunately, we can't use the `Url` from `servo/rust-url` until `rust-http` starts using it. When I tried to I got weird name errors I couldn't resolve.

```
src/request.rs:36:26: 36:29 error: mismatched types: expected `url::Url` but found `url::Url` (expected struct url::Url but found struct url::Url)
src/request.rs:36                     url: url,
```

Mostly just want to see what you think of the host + path conversion. Don't merge yet.