Adding cache support to version 0.10.0

It's a new implementation of cache based on ActiveSupport::Cache.
The implementation abstracts the cache in Adapter class on a
private method called cached_object, this method is intended
to be used on Adapters inside serializable_hash method in order
to cache each instance of the object that will be returned by
the serializer.

Some of its features are:
- A different syntax. (no longer need the cache_key method).
- An options argument that have the same arguments of ActiveSupport::Cache::Store, plus a key option that will be the prefix of the object cache on a pattern "#{key}-#{object.id}".
- It cache the objects individually and not the whole Serializer return, re-using it in different requests (as a show and a index method for example.)

lib/active_model/serializer/adapter.rb

@@ -50,6 +50,20 @@ module ActiveModel
         json[meta_key] = meta if meta && root
         json
       end
+
+      private
+
+      def cached_object
+        klass = serializer.class
+        if klass._cache
+          _cache_key = (klass._cache_key) ? "#{klass._cache_key}/#{serializer.object.id}-#{serializer.object.updated_at}" : serializer.object.cache_key
+          klass._cache.fetch(_cache_key, klass._cache_options) do
+            yield
+          end
+        else
+          yield
+        end
+      end
     end
   end
 end