Review form and	model changes.

* Get prof_course option keys from form	rather than iterator
  to avoid the problem where they don't	match.
* Remove the empty --------- choice from the prof_course field
  when the form	   is initialized.   We don't want to give it
  blank=False in the model, which would	choose a value to put
  in the form by default.
* Use first and last name in Professor slug to avoid conflicts