Added several features:

 * Message now has a new attribute number which represents the original
   packet number.

 * TCP connections are now indexed by a socket (tuple of address and
   port) instead of just addresses

 * Returned messages are now sorted according to their number attribute
   instead of just being dumped in the order they are flushed

 * A new argument called strict has been added to load_pcap to switch
   between HTTP-style handling of reassembly and strict ACK based
   reassembly