Feature requests: get url relative to another url

I just started using furl in a project. The only thing I miss a method to get the url relative to some other url. Maybe i did not find the way to do it, but i assume this is a feature request.

Something like this …
```
>>> furl("http://a.com/a/b/c").relative_to("http://a.com/a/b/d")
furl('../d')
```

... while keeping the domain in mind …

```
>>> furl("http://a.com/a/b/c").relative_to("http://b.com/a/b/d")
furl('http://b.com/a/b/d')
```

This would be a grad addition.