RFR - Adding formatting of log messages using kwargs.

Not currently working. WIP.

Whether I return at the end or return immediately (no changes) from _format_event, I get:

Traceback (most recent call last):
  File "/data/dev/teeth-overlord/teeth_overlord/tests/unit/service.py", line 12, in test_format_event_basic
    logged_msg = log.msg("hello {word}", word='world', more_than_a_string=[1, 2, 3])
  File "/data/dev/teeth-overlord/devenv/local/lib/python2.7/site-packages/structlog/_base.py", line 170, in _proxy_to_logger
    args, kw = self._process_event(method_name, event, event_kw)
ValueError: too many values to unpack

If I run the tests without configuring the processor, it works fine.