[COOK-2051] Restore postgresql.conf commentary and add optional postgresql::config recipe

A few improvements to the existing cookbook. There should be no change in functionality with these changes, but there are some very nice-to-have additions:

(1) Careful about Debian minor version numbers. The current Debian stable version is 6.0.6, so testing "when node['platform_version'].to_f == 6.0" is probably incorrect. I believe it is safer to use tests such as "when node['platform_version'].to_f < 7.0" in a chain of debian version tests.

(2) Restored helpful commentary to the postgresql.conf template, still using the key in node['postgresql']['config'] attributes as before. This will be very helpful with supporting the postgresql-9.X releases from the same unified templates. Note that I also added <% if node['postgresql']['version'].to_f < 8.4 -%> blocks to wrap new/old/modified postgresql.conf parameter changes. This file is fairly stable between releases, but it's not static. There are some bigger changes coming with 9.X.

(3) Optional postgresql::config recipe to install useful administration tools to support postgresql::server. This is the contrib directory of the PostgreSQL distribution, and it is maintained by the same development team as the PostgreSQL server. These include porting tools, analysis utilities, and plug-in features that are not part of the core PostgreSQL system, but that database engineers often require. Some (like pgbench) are exeuctable. Others (like pg_buffercache) are installed into the database. The recipe is almost trivial, and I left it purely optional to include along with postgresql::server if desired.