New non-interactive backend: produces .emf files

I just submitted a patch to add a new non-interactive backend that
produces enhanced metafiles, an OpenOffice and Microsoft Windows
scalable graphics format that can also be embedded in .rtf files.

The backend is based on my pyemf package that I finally released with
its first public beta:

The API should be stable -- I'm only planning on adding to it and not
changing any existing method signatures in the version 2.0.* series.

Oh, and I didn't say so in the patch itself, but I'm happy to donate
the patch to matplotlib under the default matplotlib license.