Added intercepting function for calls to sys.setrecursionlimit()

It was possible for students to set the stack size to something very
small (so small that other Python modules would cause a stack overflow
during normal operation). socrates now protects the stack size from
getting any smaller than it is by default.
Merge pull request #12 from abreen/devel

Added intercepting function for calls to sys.setrecursionlimit()