Missing 'object diff' method

It seems the api is missing the object diff method. As easy as:
```
def object_diff(self, hash_a, hash_b, **kwargs):
args = (hash_a, hash_b)
return self._client.request('/object/diff', args, decoder='json', **kwargs)
```

Locate the existing Python client methods that wrap IPFS API endpoints and use them to find the appropriate implementation file. Add the requested object_diff method for /object/diff, then verify that the client can request the endpoint and decode its JSON response.

Written by the indexing model from the issue text.