[extra::url] Support URL with non-relative schemes

[SimonSapin]

The current implementation of the Url struct looks like this:

pub struct Url {
    scheme: ~str,
    user: Option<UserInfo>,
    host: ~str,
    port: Option<~str>,
    path: ~str,
    query: Query,
    fragment: Option<~str>
}

It always has user, host, port, path and query fields, but those only exist for URLs with a relative scheme such as mailto or data. The data structure should be something like this:

pub struct Url {
    scheme: ~str,
    scheme_data: SchemeData,
    fragment: Option<~str>
}

pub enum SchemeData {
    RelativeSchemeData(RelativeSchemeData),
    OtherSchemeData(~str),
}

pub struct RelativeSchemeData {
    user: Option<UserInfo>,
    host: ~str,
    port: Option<~str>,
    path: ~str,
    query: Query,
}