Rewrite model to put assignment/user info into submission records.
Add Course model
Change contents to messages in submission
Divide submit API into logic and NDB implementation
Move TODO into post so that submit can be tested.
Submission API test matches model changes (tests are incomplete and don't pass)
Change project to assignment in submission tests
remove date so that tests pass
Add TODO about date and testing. Tests pass.
Add TODOs to test_api.py
Minor tweaks in test_api.py.
Merge pull request #39 from Cal-CS-61A-Staff/test_api_tweaks

Minor tweaks in test_api.py.
Rewrite model to put assignment/user info into submission records.
Add Course model
Change contents to messages in submission
Divide submit API into logic and NDB implementation
Move TODO into post so that submit can be tested.
Submission API test matches model changes (tests are incomplete and don't pass)
Change project to assignment in submission tests
remove date so that tests pass