Reuse code from my tsermon.py tool to get control node event loop quickly.

- Feeling very hurried here. I'll probably rework it later.
- Still need to implement network transparency.
How did I forget to include a proper header in sensor_node.ino
Whoops. Move the sounder to pin 8.

(I was operating from an incomplete chart and didn't notice that Pin 10
 was also used by the Ethernet shield.)
Abstract the method used to write JSON packets.

(Prepare for network transparency)
Add a "node_id" JSON parameter so the master need not track packet source
Implement support for communication via the Arduino Ethernet Shield

(Uses explicit SD card slot disabling trick from
http://forum.freetronics.com/viewtopic.php?f=4&t=176&start=50 to make
Ethernet shield operate reliably without the SD card library loaded.)
No need for urwid. Rewrite main() to use select() and listen to UDP.
Bring main.py up to absolute minimium architectural spec.
Implement a rules system and a rule to turn off lights when it's bright out

("Controller Goal 1" in the initial concept listing)
Add the "Fork me on GitHub" ribbon.