Implemented deleting Classes.
Fixed bug when adding prereqs.

Any prereqs added to a set without any prereqs were getting deleted
immediately after being added.
Moved JoinClass action into ClassController.
Merge pull request #73 from jwien001/master

Deleting classes and a couple of bug fixes.
Students can no longer add a Problem to a Set.
Merge pull request #74 from jwien001/master

Students can no longer add a Problem to a Set when creating a problem.
Added several PyInterpret features

Time limit to code runtime, recursion depth protection, and professor
code handling