Merge pull request #59 from openwebwork/master

update freeze/thaw   to nfreeze/nthaw
Allow SetMaker2 to work with library version 2.5
Commit mathjax changes
Add mathjax changes
update mathjax
Change version number to 2.5.2.1

This version has a different database schema.  It's possible to upgrade courses to this  but
it might not be so easy to move them back to this.
Update themes

Move conf/templates to conf/old_templates to avoid accidental references to old files

Add link to code mirror to math, math2, math3
Update README's and LICENSE
Preperation for merging to mgage
Add back ProblemGrader.pm and ShowAnswers.pm -- files needed for the new essayQuestion

This also revises how past answers are handled. They use the database instead of a log file.
Add remaining changes needed to enable essayQuestions

This also changes the way past_answers are stored. The are now stored in the database.
Set version number to 2.5.2
Fix out of place brace
Bug fixes and minor upgrades to Essay Answers
Link codemirror2 to htdocs/codemirror2

This avoids including codemirror2 in each theme.  This change
was made previously but was overwritten in a subsequent update.
Replace publish by visible in database fields

This will cause some incompatibility for old courses.  It may require you
to reset whether a homework set is visible or not (once).  It will
also cause error messages in upgrade courses about extra fields in the database
which are not in the schema (e.g. published) -- these can be ignored.
Restore missing lines around 1955 that fetch user problems for a single user

These lines seem to have been inadvertently removed while updating the default structure.
Use Storable::nfreeze instead of freeze.
This fixes abug discussed in http://webwork.maa.org/moodle/mod/forum/discuss.php?d=2562
remove codemirror2 -- not ready yet for prime time
remove more codemirror files for now