The example can now handle empty commands
Cleaning up and misc. fixes
Updating lastActivityTime in PEX _PeerRecord

lastActivityTime is now updated whenever a message is received
from them.  It is now useful for implementing PEX utilities.
Added PEX payload encoding and decoding

I also modified the _addrToPeerId to make sure it used the dot
notation of the IPv4 address instead of any host name.