copy is a deep copy operation, which differs significantly from dict

`Munch.copy` performs a deep copy, while [`dict.copy`](https://docs.python.org/2/library/stdtypes.html#dict.copy) is shallow. I think it would be better to implement `__deepcopy__` and let users use the [`copy.deepcopy`](https://docs.python.org/2/library/copy.html) function.