Single file built

Hi @sonoisa, 

I've created a single-file built of XyJax for the [MathJax third party extension repository](https://github.com/mathjax/MathJax-third-party-extensions), including a compressed version using the [MathJax-dev tools](https://github.com/mathjax/mathjax-dev).

I wasn't sure how you would organize the code. Could you suggest a naming convention for the folder I added? Then l would update the pull request.

Thanks,
Peter.