Automatically generate debian package with pkgr.io

Here are the changes I had to make to generate debian packages of Gitlab for Ubuntu 12.04 and Debian 7.4, using https://pkgr.io. The installation process is currently described in https://pkgr.io/apps/pkgr/gitlabhq.

In addition to the specific stuff related to packaging, I added a new `gitlab:shell:install` to streamline the installation of gitlab-shell (with auto-generation of its config.yml file), and a way to set the Gitlab configuration using environment variables. Please let me know what you think.