Add ability to connect to a Postgres database via JNDI

Hi,

I added a copy of the Postgres record that allows you to connect by specifying just a JNDI :name. Among other situations, this is useful when you're running on the Immutant / JBoss app server and the database connections are managed by that.

This is just a quick hack accomplished by copying the whole defrecord -- we could abstract out the underlying functions, since they're exactly the same.

Best,
Paul