9740b2b added pickling support.

However, in https://github.com/dpkp/kafka-python/pull/899/files#diff-ec0c29b3ee77321d4150e7578616a894L9, I removed the import, as it appeared to be unused. As a result, the entire file 9740b2b#diff-74e9dcafb2f9d7443ee297bb5aab1c32 is never used.

So either the import needs to be added back, or the entire file kafka/protocol/pickle.py should be removed.

I haven't spend the time to research how all this works... @dpkp does importing pickle have some silent side-effect that fixes pickling?