Adds Xively logging via WiFi.

- Not particularly fond of how the xively code looks, but then again
  I've forgotten almost everything I knew about C++.  Suggestions as
  to how to make the code suck less are welcome.  (Essentially what I
  want is to only allocate as many data streams as there are sensors
  connected.  And I wish to detect the number of sensors at boot time.

- Error messages and error handling is all over the place.  Should sit
  down and clean up all the different ways sensors can become
  disconnected and how to handle it.  Rebooting is a bit brutal, but
  it should work.