Logic for MathMl; Fixed Bug; Updated Readme

Three things have been added:

1. Business logic to support MathMl. This added for user smartass, who is has created a new reader that needs this logic present.
2. Fixed a **MAJOR** bug - if Typogrify was set to False in the settings, this plugin would not work (due to a stupid declaration bug). Fixed now.
3. Updated Readme, made things a little clearer.

I don't expect to change this plugin anytime soon, unless someone finds another bug or smartass requests another feature.