Add provisional Debian packaging.

This adds the ability to generate Debian source packages by running
```
./autogen.sh; make F_METADATA_REGENERATE=1 UBUNTU_RELEASE=precise deb-src;
```
.

It may be that it's not all that helpful to harfbuzz upstream to have this in the repository, but it makes maintenance of the provisional FontForge repository easier. Let me know if there's anything we can do to make this a winning proposition for upstream.

One can change the default values for `F_PACKAGER_NAME` and `F_PACKAGER_MAIL` as appropriate to the repository.