support url generation for mounted routers

[flosse]

This is a quick attempt to solve #132 .

[sapsan4eg]

Hi could you please add to the method https://github.com/iron/router/blob/master/src/router.rs#L185 same behavior? This related problem with crate mount and redirect slash. There issue #128

[sapsan4eg]

Unfortunately this test won't passed

#[test]
    fn test_with_mount_second() {
        let mut url = "http://localhost/mounted/foo/bar".parse().unwrap();
        url_for_impl(&mut url, "/foo/:user", {
            let mut rv = HashMap::new();
            rv.insert("user".into(), "bam".into());
            rv
        });
        assert_eq!(url.to_string(), "http://localhost/mounted/foo/bam");
    } 

this is not a complete solution

[flosse]

this is not a complete solution

Ok, so I'll close this PR until s.o. found a better solution :)

[Hoverbear]

@flosse Thanks for much for your contribution anyways. :)