Yahoo! BOSS Geo PlaceFinder support

It looks like Yahoo's old PlaceFinder endpoint (http://where.yahooapis.com/geocode) doesn't work anymore for new customers. Instead, they replaced it with a BOSS Geo service (http://yboss.yahooapis.com/geo/placefinder) that requires a consumer key and secret. This pull request adds a new Geocoder that works with Yahoo! BOSS Geo PlaceFinder.