Support accessible SVG output

[gcapiel]

Keep original LaTex and formulas within SVG using desc elements and aria labels.

[zachlatta]

The above commit added rudimentary SVG support. We'll look into using desc elements and aria labels to preserve information.

[gcapiel]

@zachlatta - thanks! this is great for accessibility. you might also want to collaborate with us on adding SVG graph sonification and voicing using both the Web Audio API and Web Speech API. The Web Speech API is implemented in the WebKit nightly download, OS X Mavericks or Chrome with this flag turned on chrome://flags/#enable-experimental-web-platform-features. The Web Speech API also works in iOS 7, but we haven't mobile optimized the sonification controls.

You can check out a prototype of our work at:
http://www.svg-sonifier.com/MathTrax/ and upload this image: http://www.svg-sonifier.com/MathTrax/examples/parabola1.svg

We plan to generalize the work, so you can apply it to any graph SVG that has semantics, which indicate the data line and other information.