New modular plugin structure for quiz reports.

New modular plugin structure for quiz reports.

The code for reviewing an existing attempt is now separate
in review.php and now has a log entry of it's own.

The overview and regrade reports are now in separate subdirectories
under the "report" directory.  Each has a primary "report.php" file
which implements the report as a class.

These existing reports are very simple, but now more complex ones
can easily be written.  (I am about to do one).