Addition of listener and buffer for new emoncms node interface

Hello Jerome

This is a new listener and buffer to work with the new emoncms node interface that needs to oem_gateway to just forward the RFM12 Demo output on without conversion to integers to this API:

http://localhost/master/node/set.json?nodeid=10&data=20,20,20,20

where each comma separated value is a byte value.

There's a lot of repetition with the standard listener and buffer not sure if the differences could be abstracted, configured out somehow. What do you think? Is it worth it?