Added pkgin support to Salt

Here comes pkgin support for Salt. Pkgin is a package manager http://pkgin.net/ which uses binary packages created with the pkgsrc framework. It is used by NetBSD, DragonFly, Joyent's SmartOS (based on SunOS), Minix and Mac OS X.
The pkgin.py Salt module supports all basics operations but has a limitation regarding file_list, at the moment, pkg.file_list can only take one argument, I'll work on this.