sort module names intelligently (i.e., "Week10" comes after "Week9")
only remove Problem from session (on exit from dev page) if quiz

This fixes a bug where the "Save and test" button on the
EditProblemPage only worked once because the DevelopmentPage's
PageBackHandler was unconditionally removing the Problem from
the session.  Changed it to only do this if a QuizInProgress
object is present in the session.
always add saved problem to session

This ensures that when the "Save and test" button is clicked,
it takes the user to the problem he/she is editing, not whatever
problem happens to be hanging around in the session (which
is not necessarily the one being edited if the edit problem
page was reached by creating a new problem.)
Merge pull request #50 from daveho/master

additional changes to make "Save and test" button work reliably