Add support for custom HTTP request headers

This addresses issue #195.

Add support for cache stores with :get and :set methods.

Altered the Geocoder#ip_address? method to recognize IP addresses prefixed with ::ffff: as is common with webservers binding to ipv6 sockets

Should have been part of commit eba79f25.

Return zero results more reliably. Formerly an exception was raised when
the table didn't have an 'id' column.

This is to fix #187

Yandex service fix

In lookups/base, invalid JSON needs to be rescued for ActiveSupport::JSON

Fix for parse_raw_data -- json parse catches invalid json and activesupport::json needs to be caught.

Fix for: Issue #162

within_bounding_box scope takes field names from options

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'