Various improvements making the cookbook more flexible

Although I've added some property keys to enable connection to a SQL back-end, I have not attempted to address the "Provisioning a SQL database" steps mentioned at http://docs.datomic.com/storage.html.

So, at present, the cookbook alone will not get Datomic ready for a SQL system. I'll be looking at this process myself soon, so I'll see what works for me. Maybe I'll find some things to share back here.