Unicode-friendly BrowsableAPI and JSON renderers

As discussed here:
https://groups.google.com/forum/?fromgroups=#!topic/django-rest-framework/8aoqfnV18t4
Added two new classes to renderers.py to support Unicode characters.
As mentioned, this does not break current implementations and allows for a quick switch when unicode is needed (all you have to do is change the renderer class).
My first pull request so I hope I'm doing everything alright :)