Add latency calculation to sip pings.

[atibdialpad]

Add latency calculation to sip pings.
Display '-' whenever latency can't be calculated like in aggressive mode, 100% loss, etc

Sample outputs :

1. normal case :
   /usr/local/sipping/sipping.py -c 5 -S <source_ip> -p 5060 -q -d <destinatio_ip>

--- statistics ---
5 packets transmitted, 5 packets received, 0.0% packet loss, 48.74 ms avg latency

2. aggressive mode
   /usr/local/sipping/sipping.py -c 5 -S <source_ip> -p 5060 -q -d <destination_ip> -a

--- statistics ---
5 packets transmitted, 0 packets received, 100.0% packet loss, - ms avg latency

[atibdialpad]

Hey @pbertera can you please have a look ?