Crude debian source packaging

As mentioned in #1100 I wanted to be able to generate a .changes file, together with a source package that can be uploaded to a "proper" debian repository along with the binary file. "Proper" is probably a misnomer in this case, since the packaging of the source package just barely passes scrutiny (change log is sparse, no actual build rules that can generate a binary package, even if you download it, it would require an internet connection to build), but at least you would be able to push this to a repository, download it and with a few manual steps generate the binary package.

This pull request will not enable you to get Heka included in the official Debian repositories, but I figure that is not very high on your list of priorities either. I however would like to get it included so I don't have to maintain a merge-branch to use Heka in my own debian repository. It would also enable you to host Heka in your own Debian repository should you wish to do so.

The CMake module I use for this is taken from another project called OpenRave, you can find the original file here: https://github.com/rdiankov/openrave/blob/master/modules-cmake/DebSourcePPA.cmake and you'll notice I did some modifications to it to get it to work properly for Debian rather than PPA:s.