Tweaking definition of Electricity class

* use namedtuples for DataFrame columns to make it easier to parse
* represent multiple appliances measured on a single `appliance`
  channel
* rename `map_appliance_to_room` to `appliances_in_each_room`
* `appliances_in_each_room` maps from rooms to a list of appliances
* `meters` now maps from a standard (<meter manufacturer>, <model>)
  tuple to a list of all the channels which use that meter.  The
  intention is that we'll have a `meters.json` file somewhere which
  lists all the meter makes and models that we care about, along with
  some useful metadata.
* The doc string for `appliance_metadata` has been updated to reflect
  our discussions on issue #40