Add first draft of emonTH gas analogue monitoring sketch.

Hi Glyn,

See attached for analogue pulse sketch using emonTH. I've just updated it to use ACKs and retries to be a little more robust, and roll the pulse counter on to the next monitoring 'session' if all the retries fail. This should significantly reduce pulse loss to network issues, although in fairness I've not tested the RFM12b ack/retry code (it works OK first time :-)