Move fetching last known offset logic to a stand alone function.

The `Consumer` class fetches the last known offsets in `__init__` if
`auto_commit` is enabled, but it would be nice to expose this behavior
for consumers that aren't using auto_commit. This doesn't change
existing behavior, just exposes the ability to easily fetch and set the
last known offsets. Once #162 or something similar lands this may no
longer be necessary, but it looks like that might take a while to make
it through.
also reset `self.fetch_offsets` in `fetch_last_known_offsets`
Add pylint to tox.ini; test both kafka and test; default to error-checking only; fixup errors; skip kafka/queue.py
Merge pull request #177 from zever/fetch-last-known-offsets

Move fetching last known offset logic to a stand alone function.
Add test timers via nose-timer plugin; list the 10 slowest by default
Raise TypeError in kafka.producer.send_messages if any msg is not a str (or subclass); document
Add producer unit test (test/test_producer.py); check supported types in send_messages
Merge pull request #211 from dpkp/str_only_produce_msgs

Strictly enforce str message type in producer.send_messages
Merge pull request #208 from dpkp/add_pylint_to_tox_ini

Use PyLint for static error checking