Add a function to get db connection parameters

Get them from the environment var DATABASE_URL.
If that doesn't exist, squawk.
Test the connection, and if it doesn't work, squawk.
Postgres test is pretty happy

implemented the full test for Postgres including releasing the
backend resource.
Before this is complete it needs
- file and mem backends to implement remove!"
- property for "what happens when you call swap! or reset! after
release!"
(and maybe also double-releasing, which could please not fail)