Account Domain
An Account is a representation of a valid account for an Entity. Only entities with valid accounts may login to a system managed by accounts.
An AccountRequest represents a request for a new account. When the account request has been approved, the account request will be deleted and an Account will be created.
A Credential is a representation of a user's login id and password. The password must be encrypted.
I'm considering making the Credential abstract to allow for credentials of other types.