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.
I'd rather get the secret from openproject itself. We have a rake task for that

````bash
$  bundle exec rake -s secret
require 'rails/all'... 0.360s
Bundler.require... 1.290s
afe678ba430b8b696273787aa1b30813531f581eba7e9b3a31f6297e99f245eb98f6b2c2d7be6b2419b3a0532ada7d52f81442639ac61c40f2e5a6fa7ad66444
````

I've just seen that the output of that task is a little verbose. We'll move the first two lines of output to `stderr` in a follow-up PR soon. (for now you can use `bundle exec rake -s secret | tail -1`)