Return a mapping of implemented HTTP methods vs. their callbacks.

def implemented_methods(cls):
        '''Return a mapping of implemented HTTP methods vs. their callbacks.'''
        if cls.__implemented_methods:
            return cls.__implemented_methods
        cls.__implemented_methods = {}
        for method in cls.callbacks:
            for op in getattr(method, 'swagger_ops'):
                cls.__implemented_methods[op] = method
        return cls.__implemented_methods