mathtext patch

I got the SVG backend to work properly. The PS backend seems

    > to be very tightly integrated with the current
    > implementation of the Fonts classes so I haven't realy
    > digged into it yet.

    > This patch only adds four new files for converting TeX/Type1
    > <-> Unicode, and the modified file.

A couple of comments

  * can we put all the dictionaries and data into a single file
    (_mathtext_data). I would rather not have a proliferation of
    support modules for mathtext

  * do we really need dictionaries for tex2uni and uni2tex. Would it
    not be better to define tex2uni and then do

     uni2tex = dict([(v,k) for k,v in tex2uni.items()])