Flow tracker now writes output to log file, and a benchmark_terminator module was added.

New example command to launch POX:

./pox.py samples.pretty_log openflow.discovery
misc.groupflow_event_tracer openflow.igmp_manager openflow.groupflow
openflow.flow_tracker misc.benchmark_terminator log.level --WARNING
--openflow.igmp_manager=INFO --openflow.groupflow=INFO
--openflow.flow_tracker=DEBUG --misc.groupflow_event_tracer=DEBUG
--misc.benchmark_terminator=INFO