Make Gogs packageable with https://packager.io

Currently only enabled for ubuntu-14.04, since the dependency on git > 1.8 rules out other distributions.

Current packages available from: https://packager.io/gh/pkgr/gogs

Full system test is running on AWS based on the following code: https://github.com/pkgr/showcase/blob/master/data/gogs.sh.erb

Screencast demo available at http://www.youtube.com/watch?v=xllP7BP_qgs&feature=youtu.be