Add new “salt.pkgrepo” state

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
Merge pull request #75 from rhertzog/pkgrepo

Add new “salt.pkgrepo” state