A few 'tweaks'

Hi Daniel
Not sure entirely about these. though I think they provide minor improvements. So, merge, or not, as you see fit ;-)
Uses longer dash − rather than minus - for better visibility in editor interface
uses \frac rather than \over for fractions

Appreciate this is a work in progress, but I have been testing configuration.html with some more complex TeX and mhchem expressions.

So, have added handling of a few more MathML tags: msub, munder, mover and munderover.

However, still does not yet deal with some more complex patterns of MathML tag attributes such as:

<mspace width="-.1667em"></mspace>
<mstyle mathsize="0.85em"></mstyle>
<mpadded depth="0" height="0"></mpadded>
<mstyle displaystyle="false" scriptlevel="0">

etc