Modify user access and passwords

At present the only way to give a user access to a database is to create the database first, and the user second, with the database as a parameter in the create call. We need the ability to both grant and revoke permissions to databases to users. To simplify things, both the user and the database must already exist.

In addition to modifying user access, we also need an API call to change the password of an existing user. At present, the password is set only at the time of the user's creation.