Addition of flush calls to pi_iptables_api

Added the API calls:
Flush specific chain : cgi-bin/toolkit/pi_iptables_api.py?flush={chain_name}
Flush all chains : cgi-bin/toolkit/pi_iptables_api.py?flush=all

The calls are implemented in the 'flushRules' method in the IPTablesManagerAPI class in 'pi_iptables_api.py'.