Re-factor pgd_cookbook for Chef-CentOS

refs issue #5

Remove Ubuntu functionality. Default cookbook is now a Centos-6.5 box.
yum-ius, yum-epel, yum-osuosl are now included cookbooks; yum and apt
cookbooks have been removed.

Cookbook depends on build-essential, yum, and yum-ius. No longer depends
on ubuntu and apt.

_centos.rb recipe has been added to setup centos machine. This file was
taken from the whats_fresh cookbook.

Remove Vagrantfile. Usernames are still 'Vagrant' because they don't need
to change at the moment.

bootsrap_user.rb has been removed because it served literally no purpose.

Minor key changes were made to database.rb recipe. settings.py which
didn't exist now symlinks to pgd/settings.py. This is an issue and will
be addressed later.

A few other miscellaneous centos-compatibility and naming fixes were put
in.