Added some basic rendering functionality to past answers via mathjax.

(It will render essay answers correctly, but since it can't tell what answers
 come from what type of problem it won't render equations that aren't already in backtics or tex identifiers)
Fixed a bug where certain unrendered formulas didn't get displayed correctly
Fixed some formatting issues with achievements
fixed unintended change
Merge branch 'develop' of http://github.com/openwebwork/webwork2 into showrend
Fixed bug where Mark Correct didn't work on Problem Grader page
Two main changes

- Formulas are rendered on the problem grader page using mathjax

- Formulas and any mathmode stuff coming from essay answers are rendered
  on the Show Answers page.
Merge branch 'essayencode' into showrend
Changed ProblemGrader so that essay answers are rendered with MathJax now, instead of using pg.
Fixed minor bug with newlines
minor css tweak
Added ability to use (limited) regular expressions in ShowAnswers.pm
Added a description to the Show answers page with some documentation
Merge branch 'develop' of http://github.com/openwebwork/webwork2 into showrend
Updated RegExp stuff to be a bit more robust
Added some documentation for the more fancy wildcarding and also made the table lines
in past answer only appear between answers which are "far apart".
Adding tags to achievements
Merge branch 'release/2.8' of http://github.com/openwebwork/webwork2 into itemweaks
Merge branch 'release/2.8' of http://github.com/openwebwork/webwork2 into itemweaks
Merge branch 'release/2.8' of http://github.com/openwebwork/webwork2 into showrend