GeoIP2 handling of missing data

fix issue with empty? doc[‘locations’] on ESRI lookup for some addresses

sometime geoip database does not return complete hashes, use this
method to provide bulletproof retrieval

stacktrace example:

/gems/geocoder-1.2.6/lib/geocoder/lookups/esri.rb:24 in "results"
/gems/geocoder-1.2.6/lib/geocoder/lookups/base.rb:47 in "search"
/gems/geocoder-1.2.6/lib/geocoder/query.rb:11 in "execute"
/gems/geocoder-1.2.6/lib/geocoder.rb:20 in "search"
/gems/geocoder-1.2.6/lib/geocoder/stores/base.rb:111 in "do_lookup"
/gems/geocoder-1.2.6/lib/geocoder/stores/active_record.rb:259 in "geocode"

Improve request.location(), add request.paranoid_location()

Add Geocoder::ServiceUnavailable to the readme

The `http_client` method returns either Net::HTTP or Net::HTTP::Proxy.
The enclosing `start` block handles the proxy so there's no need to ever
use Net::HTTP::Proxy::Get.new. It's also slightly awkward to refer to
the parent object of the block (for lack of a better phrase) from within
the block.

Update readme for :maxmind to specify that it is the legacy web services.

Update readme for :maxmind to specify that it is the legacy web services not the GeoIP2 Percision Service

Force specfify ip_lookup on search, even when ip_address? fails