Samsa

Features
- Added support for python 3.5
- Added iteration to the `BalancedConsumer`
- Disallowed `min_queued_messages<1` in `Producer`
- Made `SimpleConsumer` commit offsets on `stop()`
- Supported `None` in message values

Bug Fixes
- Improved `BalancedConsumer`'s handling of an external `KazooClient` instance
- Fixed `kafka_tools.py` for Python 3
- Removed the unused `use_greenlets` kwarg from `KafkaClient`
- Improved `Cluster`'s ability to reconnect to brokers during metadata updates
- Fixed an interpreter error in `conncection.py`
- Fixed failure case in `Producer` when `required_acks==0`
- Fixed a bug causing `SimpleConsumer` to leave zombie threads after disconnected brokers
- Improved `SimpleConsumer`'s worker thread exception reporting
- Simplified `SimpleConsumer`'s partition locking logic during `fetch` by using `RLock`
- Fixed `SimpleConsumer` off-by-one error causing lag to never reach 0

Miscellaneous
- Switched from Coveralls to Codecov for converage tracking

Features
- Rewrote the producer in an asynchronous style and made small breaking changes
to its interface. Specifically, it doesn't accept sequences of messages
anymore - only one message at a time.
- Made the entire library compatible with python 3.4, 2.7, and PyPy, and
adopted Tox as the test runner of choice.
- Allowed the socket source address to be specified when instantiating a client
- Started a usage guide and contribution guide in the documentation

Bug Fixes
- Fixed unnecessarily long rebalance loops in the `BalancedConsumer`
- Fixed hanging consumer integration tests
- Fixed a bug causing the client's thread workers to become zombies under
certain conditions
- Many miscellaneous bugfixes