Move geocoders to geocoders directory to match module name
Remove Geokit based methods. Use class directly.

e.g. Geokit::Geocoders::Geocoder.yahoo_geocode was available.
     Now only Geokit::Geocoders::YahooGeocoder
Simplify return statements

 * reverse_geocode keep res unless nil (consistent with geocode)
Simplify proxy code
Replace and/or/not with &&/||/! and clean surrounding code

 * Removed unnecessary self, assignment within if
Remove unnecessary returns and simplify else case
Extract classes into own files

 * Standards have files in modular structure
   e.g. Geokit::Polygon should be in lib/geokit/polygon.rb
Correct coverage status url
Remove duplicate google group link
Add newly supported geocoders and move to top as this is important
Remove no longer relevant install script info
Merge remote-tracking branch 'josacar/master'