Transactional Producer support

New features:

• Add full support for V2 format messages with a Cython extension. Those are
  used for Kafka >= 0.11.0.0
• Added support for transactional producing (issue #182)
• Added support for indempotent producing with enable_idempotence parameter
• Added support for fetch_max_bytes in AIOKafkaConsumer. This can help limit
  the amount of data transferred in a single roundtrip to the broker, which is
  essential for consumers with a large number of partitions

Bugfixes:

• Fix issue with connections not propagating serialization errors
• Fix issue with group=None resetting offsets on every metadata update
  (issue #441)
• Fix issue with messages not delivered in order when Leader changes (issue
  #228)
• Fixed version parsing of api_version parameter. Before it ignored the
  parameter