[extra::url] Support relative URLs

The module currently does not support parsing relative URLs such as `../foo.html`.

There should be an additional parsing function that takes an optional parsed base URL:

``` rust
pub fn parse(url: &str, base_url: Option<Url>) -> Result<Url, ~str> { /* …*/ }
```

(The current `from_str` function would just call `parse` with `None` for the base URL.)
