Add user and group wrappers
    
    New classes for user and group names provide a convenient way to access
    the uid and primary gid of a user / gid of a group. The classes also
    provide chown() and chgrp() methods to simplify common operations.
    
    The wrappers are subclasses of builtin str type and behave like ordinary
    strings with additional features. The pwd and grp structs are retrieved
    once and then cached.
    
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>