'Add a single value for the given attribute type to the
message. If there are already values specified for this type,
this value will be sent in addition to the values already
specified.
@param type_uri: The URI for the attribute
@param value: The value to add to the response to the relying
party for this attribute
@type value: unicode
@returns: None'

def addValue(self, type_uri, value):

    try:
        values = self.data[type_uri]
    except KeyError:
        values = self.data[type_uri] = []
    values.append(value)
