[extra::url] Support relative URLs

[SimonSapin]

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:

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.)