SendUnicast() function and supports (#18)

[ajanhalt]

This adds the SendUnicast(IPAddress) function to implement unicast responses. There are two supporting functions : getRemoteIP() to get the source IP address and getDestinationIP() to get the target IP address (if you need to see if the original packet was sent unicast for some reason). A new example shows a simple use of those for a host address responder Some fixes in the library code