allow multiple passwords per user to enable credential rotation

original report: https://github.com/gholt/swauth/issues/24

In order to change a user's password when the clients are distributed and the system is under production load, it is useful to be able to manage multiple passwords per user, to enable a phased change.

* create new password
* change clients configuration
* revoke old password

Amazon describes this process here: http://docs.amazonwebservices.com/AWSSecurityCredentials/1.0/AboutAWSCredentials.html#CredentialRotation