Fix `\\(...\\)` math rendering in preview.

Reverts 5ed4dbf90315b9e890305d2be3f5532e27b14636.
Added displayMath for self-containment (equal to mathjax default).
Make Pandoc recognize double-backslash delimited math (#46).
Mostly improves latex (and therefore PDF) output which otherwise typesets `\(` etc instead of treating it as math.

Load MathJax in HTML exported from Pandoc.
Don't specify HTTPS mathjax CDN address because it might change soon
https://groups.google.com/d/msg/mathjax-users/cPD_EI_lFAw/ovo4Bv5pidoJ

Pandoc's HTTP default, while less secure, is unlikely to break and will hopefully change to new https address if it emerges as a future-proof option.
Use \lim in example display formula to demonstrate display-style limits.
(Also fix inconsistent differentiation variable)
Merge pull request #47 from cben/master

Improve math handling.