Make MathJax a dependency

Fixes #42 by turning a tarball of a slim branch of MathJax into a dependency.

Work in progress

I haven't figured out yet how to replace the current harcoded `MathJaxPath` with something using `require` (e.g., using [require.resolve](http://nodejs.org/api/globals.html#globals_require_resolve)). If somebody has any pointers, please let me know.