Added firewalling option

When Using P.create of node.connect an optional
firewalling function can be passed via the "firewall"
key to firewall RTC requests. This will reject
any connections for which the firewall function
returns false.

Signed-off-by: Ozan Turgut <ozanturgut@gmail.com>
Added firewalling example

Signed-off-by: Ozan Turgut <ozanturgut@gmail.com>