Finished up py.test Redis plugin.

* Output is now highly snazzy and should be very useful moving forward.
* The pytest_redis.py module is a full-fledges py.text plugin and
  should be treated as such.
Added top-level conftest.

* This file adds the --redis command line argument required by the
  py.test Redis plugin added in the last commit. The reason this has
  to be done here is that the command line arguments aren't loaded
  in time if I put the definition in the plugin file.