ProblemSet view now displays Prereqs.

Adding a new Prereq also works on the client side, but is not currently
hooked up to the backend.
Figured out how to overload controller actions.

Simplified the execution flow for saving a problem set.
Can now add Prereqs to the View list.
Implemented deleting Prereqs from the list.
Prereq changes are now saved in the database.
Created a separate ProblemSet controller.
Merge pull request #40 from jwien001/master

Implemented CRUDing Problem Set prerequisites