add bsn_log extension

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.
bsn_log: add a comment about the string format
Merge into master from pull request #288:
add bsn_log extension (https://github.com/floodlight/loxigen/pull/288)