Debian packaging with pkgr

Hi,

Reading <http://meta.discourse.org/t/installing-discourse-on-debian/2349>, I thought it would be nice to make a proper debian package for discourse. For that I'm using [pkgr](https://github.com/crohr/pkgr), which allows to build a package using the ruby heroku buildpack abd the [fpm](https://github.com/jordansissel/fpm) tool. Basically this means that if discourse works on heroku, it can be packaged, and it will embed all necessary ruby / gem dependencies.

I only had to make a few changes to make it happen, especially regarding SMTP settings, and renaming (in the Procfile) the `sidekiq` process to `worker` to be more in line with what's expected from the buildpack. 

The result can be found at <http://deb.pkgr.io/crohr/discourse/>, along with installation instructions.

Please let me know what you think.