Custom MathJax rendering script option

For some reason path to `MathJax.js` has been hardcoded, so I made the `mathjax-script` option in config to make it possible to specify the path to the script.

I am not sure if there is confusion in my description of `mathjax-script` option in the `default.conf` file: you have to either run your own web server (not that hard, but anyway) or copy the entire MathJax directory into gitit's `data/static/js` directory (which is not the way it is intended to be done, I guess), so there is no per-wiki directory for custom js files?

P.S. Why is some random blahblahblah.cloudfront.net being used, not cdn.mathjax.org?