RFM2PiGateway: make logfile and PID file optional

RFM2PiGateway class usability improved: PID file and
logfile not created at __init__ but at instantiation.

Sleep time reduced to 200 ms for increased reactivity.
RFM2PiGateway: Add command line arguments

Make rfm2pigateway.py accept command line arguments:
--no-log-file    log to Standard error stream STDERR (default: log to file)
--show-settings  show RFM2Pi settings and exit (for debugging purposes)