Enhance to support MathML

Hi, first let me say, thanks for the excellent piece of software!

I've done a good bit of work on it over the last couple of days.  My code isn't nearly as elegant as yours, and my changes are extensive, but I hope if you look them over, you will like them.

I kept backward compatibility in all but one respect:  you now have to download and install mathjax locally.  Unfortunately, I don't think there's a way around that, because the version they have deployed on the CDN is broken -- see my comments in the README.md and index.html.  If you try to use the CDN version, it crashes phantomjs.

Other than that, here's the list of changes:
* Got it to work with MathML.  To do that, I had to add another "input jax", which means a new \<div> on 
  the page.  Then, to distinguish that from the latex one, I parameterized a bunch of stuff,
* Added the ability to do conventional query-string parameters (or html form fields via POST).  It is still backward compatible with the way you had it:  so GET without any params is assumed to be URL encoded source string, and POST without any params is assumed to be raw, un-escaped source string.  If there are any params, though, then the whole thing must be properly escaped (GET or POST) -- because otherwise there would be no way to parse out the params.
* Added the "width" param, that allows you to constrain the width.
* I added some more error handling.  In particular, if MathJax craps out and doesn't even generate an <svg> element, that used to crash the server, and now it doesn't.