No conversion for Internet Explorer

I modified the convert routine to keep the MathML code as it is if the browser does not support certain XML document object interface call. Internet Explorer is known not to support that call. So, the convert routine will not do anything if the browser is Internet Explorer.

This is because IE can already render MathML with the aid of Mathplayer. We only need to convert (actually recreate) MathML code for Firefox. This has a certain amount of performance issue. So, why penalize IE users when it can be avoided.