Beefed up traffic sniffer
* modules/sniffer/traffic_sniffer.py
  -- Added a filter option to specify what traffic is sniffed
* modules/util.py
  -- Added a function for verifying the syntax of a tcpdump filter
* scapy/arch/linux.py
  -- Scapy doesn't read stderr when attaching filters, so we
     weren't able to take care of tcpdump complaining about invalid
	 filter syntax.  Modified slightly to take care of this.