Support counting pkt-in msgs in controller

Count pkt-in messages as they are received
Allow the message queue to be cleared with new clear_queue fn
Also small bug fix: deref null exp_msg
Added packet-in monitor test case

This test case starts the controller connection with keep-alive.
It then polls the controller to count packet in messages.  It
reports the number of messages received about every second.
Minor updates for pkt in monitoring

Response to comment for rename of packet count
Clarified periodic_task name
Indentation
Merge pull request #48 from floodlight/det-monitor-pkt-in

Test to monitor and report packet-in rate.  Disabled by default as it is "blocking".  Note that this only supports a single controller (determined by the command line parameters).