Upload Debian source packages to Apt repository

This patch adds support for uploading Debian source packages (*.dsc) to the Apt repository. Note that this is more complicated than uploading a single file because each source package references two external tarballs that also need to be uploaded.

Overall, this adds four steps to the build process:

1. Upload the .dsc source package to the Buildbot master.
2. Upload the .orig.tar.gz tarball to the Buildbot master.
3. Upload the .tar.gz tarball to the Buildbot master.
4. Upload the source package to the Apt repository.

I don't use reprepro to host our Apt repository, so I have now ay of testing whether the `include-repro.bash` script will properly upload source packages. It does, however, work with [aptly](https://github.com/smira/aptly).

BTW: If you're looking for a new tool for managing your Apt repository, I *strongly* recommend taking a look Aptly. It really is a pleasure to use, especially compared to the lackluster alternatives that we've tried (e.g. reprepro, dearchiver, mini-dinstall).