[COOK-2232] Provide PGDG yum repo to install postgresql 9.x on redhat-derived distributions

I am providing an optional recipe to enable the PGDG yum repo for newer versions of PostgreSQL. I took exactly the same approach that was used for the PostgreSQL backport PPA repository on debian and ubuntu. My yum_pgdg_postgresql.rb should work for node['platform_family'] = "rhel", "fedora", or "suse". 

This depends on:
   http://tickets.opscode.com/browse/COOK-2231
   https://github.com/opscode-cookbooks/postgresql/pull/34