Option of pre-timestamped data

Currently only implemented fully in the socket interfacer. There is now
the option to override the default behaviour of automatically adding a
timestamp on arrival by using "timestamped = True" ("False" by default
when omited). This enables pre-timestamped data retrieved from modbus
devices etc to retain an existing timestamp or for historitc data to be
input via a websocket etc. Any custon interfacers can implement their
own "timestamping" or leave it to emonhub to timestamp with current
time.