Add support for namespaces to DefaultRouter.

I added a `namespace` parameter to the `DefaultRouter`. I don't know if is the best way to handle this, but doing it this way saves me from having to rewrite the `get_api_root_view` in my own code.

I don't know what recommendations you have for this. Maybe you have a different idea for how to do it, but I thought I would put up the code at least, because I wrote it in my project already.

I only ran this against Python 2.7/Django 1.7.1 locally, I will fix when Travis comes back.