Add 'codec' parameter to Producer

Adds a codec parameter to Producer.__init__ that lets the user choose
a compression codec to use for all messages sent by it.
Merge branch 'teach_producers_about_compression' into producer_compression

Conflicts:
	servers/0.8.0/kafka-src
	test/test_unit.py
Improve error handling and tests w.r.t. codecs

Add function kafka.protocol.create_message_set() that takes a list of
payloads and a codec and returns a message set with the desired encoding.

Introduce kafka.common.UnsupportedCodecError, raised if an unknown codec
is specified.

Include a test for the new function.
Merge pull request #166 from patricklucas/teach_producers_about_compression

Add 'codec' parameter to Producer