Feature/url versions

This gives the ability to use namespaces in the urls.py and have reverse pick up on them automatically so that all links generated in the api use the current namespace pattern, if any.
From this discussion: https://groups.google.com/forum/?fromgroups=#!topic/django-rest-framework/JBb-k7FTnvA

In this example the default urlpatterns is also aliased to /v1/ 

url(r'', include(urlpatterns, namespace='v2')),   # urls are like  http://example.com/user/tim 
url(r'v1/', include(urlpatterns)),    # urls are like  http://example.com/v1/user/tim 

It passes currents tests.
Let me know what it would take to get this accepted :)