Use TeXMath for equations

(I see that there are some API changes as of a few minutes ago, in 0.7 -- so I can hold off on this pull request until those are on hackage. Otherwise, it looks like it will be only a few lines to fix when it is up, just changing out `toTeXMath`, and the name of the imported modules. Let me know if you'd prefer me to wait, or to put this in now, and modify when 0.7 is released.)

This patchset gets rid of the homemade LaTeX generation and lets TeXMath do the work. The LaTeX output has improved as a result in most cases, and regressed in a few others, but those can be worked on in the library.

The minimum version of texmath has been changed to 0.6.7 in pandoc.cabal.