add bsn_log extension

Reviewer: @andi-bigswitch

This provides a mechanism for either the controller or switch to request the 
other side to output a log message, subject to the configured loglevel. This
is useful for log (and OpenFlow pcap) correlation.
I modeled these on the SwitchLight loglevels. I'm a little surprised it actually matches up with slf4j so well. I think the communication is going to be mostly controller to switch so I'd prefer to have the loglevels match what the admin configures in the switch CLI.