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.
Oh, github displayed this strangely. It's representing the fact that there is no CLRF at the end of line 60 of this file. I'm specifically referring to the content of the line:

`print "PGD Done"`.