Group resources by url

I tried to implement the "Namespaces from URL partition" feature in a more reasonable way.

My use case is the following:
I have several seperate APIViews, for example:
 ```python
    url(r'^login/token$', views.TokenLogin.as_view()),
    url(r'^login/mail$', views.MailLogin.as_view()),
    url(r'^login/facebook$', views.FacebookLogin.as_view()),
 ```

These should belong to the same resource - even if they are not in the same Viewset.

This introduces the setting *resource_url_prefix*:
 ```python
SWAGGER_SETTINGS = {
    "resource_url_prefix": "api/v1/",
}
 ```