Relative urls not being resolved

Lettuce allows you to use relative urls in your features so it doesn't matter what the server address is (e.g. "/hello_world/"). You can then use 

``` python
from lettuce.django import django_url

django_url(url)
```

to get the correct url.

Currently, lettuce_webdriver does not seem to be working with relative URLs, however this would be very nice to have implemented.
