zbwardrive improvements

[GoogleCodeExporter]

zbWarDrive desired improvements:
1. no beacon requests are sent out on channels that are currently being 
monitored
2. if traffic on a channel drops off for a specified (adjustable at cmd line) 
length of time (perhaps default 5 mins, and only if there are no open devices 
and maybe networks are pending), then stop monitoring on a channel and return 
that one to the scan list, opening up the device for use monitoring another 
channel

Original issue reported on code.google.com by [email protected] on 6 Jul 2011 at 2:16