Add modules for gitfs dependencies and documentation for how to use them.

This state downloads formulas from git repositories and ensures
that they are recorded in the file_roots setting.

This state enables the official saltstack package repository in order to
always benefit from the latest version. This state currently only works on
Debian and Ubuntu, and aims to implement the installation recommendations
of the official documentation:
http://docs.saltstack.com/en/latest/topics/installation/index.html