Represents a resource to store user information.
Implementations define the specific persistence mechanism to store data.
User data is accessed using the username as key.
author: Sebastian Gonzalez Oyuela version: $Id: IdentityStore.java 508 2008-02-18 13:32:29Z sgonzalez $
Loads user information from the store.
Parameters: key - the user name, this is a unique identifier. the found user throws: NoSuchUserException - if the user does not exist.