stack_info is not processed for python 3

[gcbirzan]

Just like exc_info is processed as a special argument for log messages, so should stack_info. stack_info should evaluate to the traceback, even without an exception (http://docs.python.org/3.3/library/logging.html#logging.Logger.debug)

An interesting question is whether this should go in as a feature for python2 (which doesn't support it natively), and I'd say yes, because we already have a hackish way of doing it currently, and it won't break backwards compatibility.

[hynek]

I love the idea of on-demand adding of tracebacks – this is absolutely going to go into a 2/3 processor.