v0.8.6

Features:

• messagebus: add mock backend (#56)

Fixes:

• messagebus.kafka: define top level config keys to use when creating producer and consumer configs (#60)
• message: keep keys with none values in to_dict method (#59)

Minor:

• bump confluent_kafka_helpers verison to 0.7.4
• make cnamedtuple an extra instead of a dependency (#55)