Flexible configuration, sensible defaults.

At least for development purposes. ;P
Don't specify release for default Debian pkgs.

This prevents problems with Ubuntu packages, where the relase name is
different.

While Debian uses `stable`, `unstable` and `testing`, ubuntu uses their
release name as prefix, like: `precise-stable` and `precise-backports`.
This information is briefly present on README now.

A `precise` virtual machine was created on Vagrantfile to test the role
properly.