I've done the following:

1.  Update the PR with @nschonni's suggestion of loading the polyfill if `!Modernizr.mathml`.  This loads the ~190 bytes of compressed polyfill and waits for `timerpoke.wb` on a `<math>` element before the MathJax payload is downloaded (gives us support for dynamic addition of elements).
1. Cleaned up the mathml.js polyfill since `window` is no longer needed.

What are your thoughts about the current bug with the `Modernizr.mathml` test?