Initial draft (non-functioning)

redesign of emonhub_listener to provide greater scope of decoding,
ability to decode any datatype to be made available to any listener on a
tiered per hub, per listener, per node or per value basis with tiered
default-type alternatives. Non-working prototype, emonhub.node access
not complete to unable to test. Initially planned for an abstract
emonhub_decoder.py but opted for the simpler method to begin with.