Add transaction context manager.
Improve documentation in example
Rename KafkaTransaction to OffsetCommitContext for clarity.
Bump version to 0.9.3-dev
Add recent PR merges to Changelog for 0.9.3
Update python version support in README
Merge pull request #217 from locationlabs/transaction

Commit/rollback consumer offsets via context manager
Separate consumers/producers/partitioners
Fixup test_context.py unittest import
Merge pull request #232 from dpkp/directory_layout

Separate Consumer/Producer/Partitioner modules
Use b''.join([]) instead of += to speedup code
Merge pull request #233 from dpkp/str_join_speedup

Improve string concatenation performance on pypy and python 3