logging: add some undocumented but used features

[tharvik]

See #247.

I still need a list of wanted attributes, but here goes the changes about the type of message.

[gvanrossum]

Honestly, I think I'd rather fix the user's code than weaken the stubs in this case. The docs say nothing about the type (they just call it "message") but the intention is pretty clearly that it's a format string using % formatting characters. The actual % operation happens in LogRecord.getMessage(), and it does call str() on the msg argument if it's not a basestring (or always in Python 3) but I'm still hesitant to decide whether this is really meant to be a feature given the lack of documentation.

Let's hold off on this until we get more complaints.

[tharvik]

The docs say nothing about the type (they just call it "message") but the intention is pretty clearly that it's a format string using % formatting characters.

In the debug definition, it is stated as "The msg is the message format string", so IMO, that's really a string and not any other object.

As we agree, I'm closing this PR.