__str__ in descriptions should go into the base class

[oweidner]

... wich is "AttributeInterface"

def __str__(self):
    '''String representation of the resource description'''
    result = str("{")
    for attribute in self.list_attributes():
        if self.attribute_is_vector(attribute):
            value = repr(self.get_vector_attribute(attribute))
        else:
            value = str(self.get_attribute(attribute))
        result += str("'%s' : '%s'," % (str(attribute), value))
    result += "}"
    return result