Add postgresql test suite

[patcon]

I've got this in another branch too, and I can push it after we merge https://github.com/bflad/chef-stash/pull/105

Woo

• [x] centos-6
• [x] centos-7
• [x] ubuntu-12.04
• [x] ubuntu-14.04

[patcon]

Note that this PR will be much much smaller once #105 gets merged :)

[patcon]

OK, everything passes! Ready to rock whenever you're comfortable doing so

[linc01n]

Give me few hours for me to go back home and have a look.

[patcon]

No hurry! It's my very late bedtime now anyhow..!

[linc01n]

:+1: Merged