Enable debian packaging with https://packager.io

This pull request adds support for packaging OpenProject for Debian 7 using Packager.io, and automatically setting up OpenProject with a wizard. It is not really meant to be merged as is, but to track the progress of the packaging process on the dev branch, and allow people to make comments.

I will extract a few separate pull requests from this branch as soon as I get back from vacation, to make the review and merging process easier.
do you set `TARGET` three times here? I guess one time would be enough :)