updated version of the PS backend


I did some clean-up of the matplotlib PostScript backend.
Main changes:

  - The generated Postscript output is now completely DSC conformant
    (I hope).

  - All PostScript definitions go into a a PostScript dictionary
    mpldict instead of the global one. This will hopefully prevent
    problems (name clashes) when including matplotlib generated eps
    files into other PostScript files

  - I moved 1.5kb of comments about drawing ellipses from the generated
    PostScript into a Python source code comment. I think it does not
    make sense to ship the comment in each and every genereated picture.

Hopefully I did not cause any new problems. Comments about the
changes are most welcome.

All the best,