Homework assignments are now listed in a more sane place

This addresses #3 

Homework assignments should now be listed under `hw.yaml` which is in the site root directory.
Mainly I think it's future-proofing. What if a professor has a grading script that takes homeworks, tests, and participation into account. Instead of having just `hw.yaml` and having to do everything else in other files, what if they had an `assignments.yml` with keys for participation, homework, etc.

Having the list nested under the top level leaves more room for growth.