grade api is operational!

I almost can't believe that the Grade API (with DRF) is fully operational with < 40 lines of code (including import statements, and docstrings). 

Make sure you do `pip install -r requirements.txt` again because I've added a couple more dependencies

Once it's installed, go to `/api/grades` to check it out (You'll want to login as the admin user, since I made this particular api admin-access required. There is a lot of customization we can do, but I wanted to start with a barebones concept. 

This is so awesome.