Added Endpoints for Smartmeter
Added Wh as valid Unit
treat socket send errors as fatal for the device and call error callback
call asio recv functions only after our handlers are dispatched

unfortunately, asio seems to violate its own documentation and *does*
call handlers from async_* functions. restarting our receives after our
handlers have run mitigates this somewhat, but sync receive will still
be broken if packets arrive back to back.
add help text to hxb-net-autoconf
fix synchronous receive dropping packets under high load
partially revert 37dd473cc4a93e810ffea1d3101359695bee5b22

that help text wasn't supposed to go away
Merge branch 'development' of ssh://github.com/mysmartgrid/hexabus into development
Fixes #239