Fix PNG rendering outside of root directory.

This commit references the `batik-rasterization.jar` file by its full
path, calculated based on the root MathJax-node directory's location.
This allows users to call the library from any directory and render
.pngs as expected.
Use path.resolve to find MathJax.js, and fix up formatting.
Don't try to find the SVG element if there isn't going to be one.
Get correct ID for title and desc elements in SVG output with speech.  Resolves issue #29.
Merge branch 'issue29'
Merge from mathjax