classRegistration: document search

Hi! I just discovered this repo. You've done an incredible job. I did some research myself for a registration script last semester. This pull request introduces the "classRegistration" tab as well as two requests, one to add a class and one to register. Unfortunately, I can't test these requests as I can't register for this summer's semester. However, the content of the queries should be right since they are taken from my notes/codes.

If something is wrong, please report it and I will correct it. Thanks a lot for these docs, I wish we could get everything in JSON :(