Ensure all do_lookup blocks have empty list protection

They return SQL but were named as if they returned simple distances.

Allow ordering by association's distances

Add the ability to order by distance in associations by extracting the distance calculation into a method 'distance_from'

Fixes issue #145

updated cli to accept google premier key

This avoids selecting all fields from all joined tables, which confuses
ActiveRecord (assigns returned object the wrong ID).

Change configuration options in the README from lat and lon to latitude and longitude

Change lat and lon configurations in README to latitude and longitude so it matches the gem's defaults and the generate migration instructions

For consistency with rest of code.