Response to GH issues #579 and #663.

This is relevant for SmartyStreets, and seems like a good idea in
general (possibly supported by other APIs).

  Example:

    class City < ActiveRecord::Base
      geocoded_by :address, :lookup => lambda{|obj| obj.custom_lookup }

      def custom_lookup
        # return custom lookup for record
      end
    end

Allow to set custom lookup option for Geocoder.search, Geocoder::Query.new and ActiveRecord::Base.(reverse_)geocoded_by

  Examples:

    class City < ActiveRecord::Base
      geocoded_by :address, :lookup => :nominatim
    end

    Geocoder.search("address", :lookup => :nominatim)

Previously it was returning an exception which is not the same behavior as the other lookups.

This commit adds the Data Science Toolkit as a lookup type.  The DSTK behaves identically to the Google Maps API with the exception that it can be run behind your own firewall.  More information about this tool can be found here: http://www.datasciencetoolkit.org/

The result of the lookup in maxmind_local is stored at test/test_helper.rb to avoid the inclusion of maxmind's database in repository.

Now it is necessary to specify which service is being used:
  Geocoder.configure(:maxmind => {:service => ...})
where '...' is one of: < :country | :city | :city_isp_org | :omni >

Now every fake query does not need to be listed in the code, a fixture
just needs to be added.

Changed since pull request was submitted.

Make Geocoder.configure accept a hash (now preferred) and add
Geocoder.config method (for reading config). Deprecate old syntax.

This requires changing the way stubs work, so that they only simulate
actual HTTP calls, not cache hits.

Add test and display Yahoo's error message in warning.