Define Debian wheezy VM on Vagrantfile.

This allows defining more machines later on and has zero impact on
previous behavior.
Debian wheezy machine is brought up by default and is considered the
primary machine.

All machines should execute the same ansible playbook.
Support PHP 5.4 on Debian squeeze.

Separates debian related tasks on a different role file (tasks/Debian.yml),
creates tasks for importing APT keys and repositories to be used for
installation.

A different varaible file (Debian-squeeze) is used after importing the
`vars/Debian.yml` file adding configurations for different repositories
to be used.

On Vagrantfile, a new machine was created using Debian Wheezy so we can
proper test the playbook on a different environment.
Document debian configuration on README.

Explaining and defining variables used for Debian environment
configuration, usefull outsite the role, like: repositories to be used
and package stability to be used.
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.
Merge pull request #9 from augustohp/gh-6_wheezy-php

PHP 5.4 support on Debian Squeeze