Add UserBuilder

[rwinch]

This will make it easier to define users without AuthenticationManagerBuidler. For example:

@Bean
public UserDetailsManager userDetailsManager() throws Exception {
    InMemoryUserDetailsManager manager = new InMemoryUserDetailsManager();
    manager.createUser(User.withUsername("user").password("password").roles("USER").build());
    manager.createUser(User.withUsername("admin").password("password").roles("USER","ADMIN").build());
    return manager;
}

@Bean
public UserDetailsManager userDetailsManager(DataSource dataSource) throws Exception {
    JdbcUserDetailsManager result = new JdbcUserDetailsManager(dataSource);
    result.createUser(User.withUsername("user").password("password").roles("USER").build());
    result.createUser(User.withUsername("admin").password("password").roles("USER","ADMIN).build());
    return result;
}
----