Add support for result caching.

Just set Geocoder::Configuration.cache to any key/value store that
implements #[], #[]=, and #keys. You can also set
Geocoder::Configuration.cache_prefix to use a custom prefix for keys.

lib/geocoder/configuration.rb

@@ -16,10 +16,20 @@ module Geocoder
     # app id (if using Yahoo geocoding service)
     def self.yahoo_appid; @@yahoo_appid; end
     def self.yahoo_appid=(obj); @@yahoo_appid = obj; end
+
+    # cache object (must respond to #[], #[]=, and #keys
+    def self.cache; @@cache; end
+    def self.cache=(obj); @@cache = obj; end
+
+    # cache object (must respond to #[], #[]=, and #keys
+    def self.cache_prefix; @@cache_prefix; end
+    def self.cache_prefix=(obj); @@cache_prefix = obj; end
   end
 end
 
-Geocoder::Configuration.timeout     = 3
-Geocoder::Configuration.lookup      = :google
-Geocoder::Configuration.language    = :en
-Geocoder::Configuration.yahoo_appid = ""
+Geocoder::Configuration.timeout      = 3
+Geocoder::Configuration.lookup       = :google
+Geocoder::Configuration.language     = :en
+Geocoder::Configuration.yahoo_appid  = ""
+Geocoder::Configuration.cache        = nil
+Geocoder::Configuration.cache_prefix = "geocoder:"

lib/geocoder/lookups/base.rb

@@ -83,11 +83,39 @@ module Geocoder
       #
       def fetch_raw_data(query, reverse = false)
         url = query_url(query, reverse)
+        key = cache_key(url)
         timeout(Geocoder::Configuration.timeout) do
-          Net::HTTP.get_response(URI.parse(url)).body
+          unless cache and (response = cache[key]) and response != ""
+            response = Net::HTTP.get_response(URI.parse(url)).body
+            if cache
+              cache[key] = response
+            end
+          end
+          response
         end
       end
 
+      ##
+      # Cache key for a given URL.
+      #
+      def cache_key(url)
+        [cache_prefix, url].join
+      end
+
+      ##
+      # The configured prefix for cache keys.
+      #
+      def cache_prefix
+        Geocoder::Configuration.cache_prefix || "geocoder:"
+      end
+
+      ##
+      # The configured cache store.
+      #
+      def cache
+        Geocoder::Configuration.cache
+      end
+
       ##
       # Is the given string a loopback IP address?
       #