Add ipalib.messages
    
    The messages module contains message classes that can be added
    to a RPC response to provide additional information or warnings.
    
    This patch adds only the module with a single public message,
    VersionMissing, and unit tests.
    Since message classes are very similar to public errors, some
    functionality and unit tests were shared.
    
    Design page: http://freeipa.org/page/V3/Messages
    Ticket: https://fedorahosted.org/freeipa/ticket/2732