PostgreSQL database present for users

This addresses #48 (closed), it doesn't do some things:

1. Support anything other than PostgreSQL database users or databases, but has been written so this could be added without a big architectural change.
2. Support the addition or deletion of additional database users (the database users created that match the account names cannot be deleted using the API).
3. Implement the PUT or PATCH HTTP methods to update existing databases (or check if they need updating).
4. Check that the all the variables are sensible (for example not allowing a request for a PostgreSQL charset / lang that isn't an valid option).

See the README.md for the syntax for defining database_users and databases arrays.