MathML polyfill

Adds the MathML polyfill based on yesterday's discussion:

1. Determine if `<math>` element exists on page before loading polyfill.
1. Use `timerpoke.wb` to trigger load from MathJax CDN.

## Issue (currently fixed by removing test element in `complete` handler)

Current release of Modernizr.mathml test leaks content into the DOM (doesn't cleanup the test element).  This has been fixed in the Modernizr repo, but still exists in their latest 2.7.0 release.  

To fix, we could add a patched mathml test to grunt-modernizr [`customTests`](https://github.com/Modernizr/grunt-modernizr#customtests-array) until Modernizr is updated.