Merge pull request #25 from goehle/ww2.5.1.3

Essay problem tweaks
Added key=>1 to fields in PastAnswer, and did some other little bugfixes.
Not sure why mathjax is changed...
Merge branch 'math4'
Merge branch 'develop' of https://github.com/openwebwork/webwork2
Made past_answers native and added more keys to improve load times
Merge branch 'develop' of https://github.com/openwebwork/webwork2 into math4
Merge branch 'fresh2.7' into math4

Conflicts:
	lib/WeBWorK/DB/Record/PastAnswer.pm
Merge branch 'math4'
Merge branch 'release/2.7' of https://github.com/openwebwork/webwork2
Bunch of things

-- Fixed a lot of broken links for js files in Problem.pm UserList2.pm ProblemGrader.pm etc...
-- Made it so that backticks work with formatting essay questions.  (Doesn't complain about variable out of context)
-- Added tabber to Achievement Editor in all themes
-- In theme math4 javascript is used to replace tabber by bootstrap tabs.  Unfortunatelly this still requiers tabber to be there and running, even though its not directly used)
-- I mostly disabled facebook integration.  It can still be used but because of the way facebook works each school that wants to use it has to create their own facebook app.
-- Fixed a bug where deleting a student didn't delete all their achievement information
Merge pull request #37 from goehle/master

Bunch 'o Things