Let logging do message interpolation.
Let logging do message interpolation.

The reason for this is twofold:
1) If someone wants to do translation of the message, they need to
 get at the raw msg pre interpolation; see the gettext module for
 examples.
2) the reason I actually care: performance.  given the following:
  `logging.debug(msg % my_arg)`, my_arg.__str__() is forced even if
  the message will never reach a handler.  However, given
  `logging.debug(msg, my_arg)`, the interpolation is performed only
  if needed- thus my_arg.__str__() is ran only when needed.

In regards to point #2, at least for my local schema making this change
is a 25% speedup in loading of a given json message when debug logging is
turned off (which is the case 99% of the time).

cwacek/python-jsonschema-objects