Complete multi-node processing

Multiple nodes can now connect. Nodes will timeout individually
based on the previous timeout values. A new ID will be given to a
new connection that hasn't been established before. All Serial.
print/println strings have now been moved to PROGMEM to save RAM.

IMPORTANT:

To get this patch to work, you must copy the files in
EthernetClientLibrary folder into the place where Arduino looks
for the library files. In OS X, this is /Applications/Arduino.app
/Contents/Resources/Java/libraries/Ethernet. I added the ability
to grab the IP address that the client connection is from in order
to differentiate between simultaneous connections.

Nodes will need to have a different MAC address in order to get a
different IP address from the router. Otherwise, the connections
will appear from the same node and will break everything.

Issue: #7