Customizeable url slugs

Allows you to define a `url_slug` attribute on the `BackboneView` subclass to customize the url (and url name). A side effect of this is that you can have multiple views registered for same model.

    # fooapp/backbone_api.py

    class FooAPIView(backbone.views.BackboneAPIView):
        model = Foo
        url_slug = 'custom_foo'


The url ends up becoming `/fooapp/custom_foo/` (and the url name becomes `fooapp_custom_foo`)