Add extra checking function to XMLRPC test framework
    
    This fixes https://fedorahosted.org/freeipa/ticket/1968 (Add
    ability in test framework to compare two values in result)
    in a general way: adding an optional extra_check key to the test
    dict, so a custom checking function with access to the whole result
    can be called.
    
    The particular test mentioned in that issue, checking that the
    uidnumber and gidnumber for new isers are the same, is added.
    
    Also, this adds a docstring to the Declarative class.
    
    And finally, the test dictionary is passed to check() via keyword
    arguments, preventing spelling mistakes in keys from going unnoticed.