Support PREA and Token in separate table from Users

[jdickey]

To avoid cluttering up the users table with fields that are only used in specific use cases, app developers may well prefer to create separate tables to support specific use cases, such as resetting/changing passwords. Configuration should be extended to add a password_reset_table attribute. Defaulting to null, it must be set to a table name if that table should be used to access the password_reset_expires_at and token attributes.