v0.11.0

This is a minimal librdkafka version-synchronized release of the Python client.

Changes:

• Handle null/None values during deserialization
• Allow to pass custom schema registry instance.
• None conf values are now converted to NULL rather than the string "None" (#133)
• Fix memory leaks when certain exceptions were raised.
• Handle delivery.report.only.error in Python (#84)
• Proper use of Message error string on Producer (#129)
• Now Flake8 clean