Revision 3377

Preliminary commit of changes to add Gateway module.
This adds to WeBWorK
 - the ability to create versioned, timed problem sets ("gateway tests")
   for which all problems are displayed on a single page ("versioned"
   means that students can get multiple versions of the problem set),
 - the ability to create sets that draw problems from groups of
   problems, and
 - the ability to create sets that require a proctor login to start
   and grade.
Sets can be defined as gateway tests or proctored gateway tests from
the ProblemSetDetail page.

Not quite bug-free yet.  Known bugs include handling of problem values
on the Student Progress page (I think this may be a problem with
changing from sql database format where all entries were 'text' to
sql_single in ver 2.1, where they are integer), and a division by zero
error on the grades page (which may be the same problem).

Tests with a number of attempts per version greater than one haven't
been carefully tested, nor has scoring of gateway tests.