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/json.rb

@@ -6,19 +6,21 @@ module ActiveModel
           if serializer.respond_to?(:each)
             @result = serializer.map{|s| self.class.new(s).serializable_hash }
           else
-            @result = serializer.attributes(options)
-
-            serializer.each_association do |name, association, opts|
-              if association.respond_to?(:each)
-                array_serializer = association
-                @result[name] = array_serializer.map { |item| item.attributes(opts) }
-              else
-                if association
-                  @result[name] = association.attributes(options)
+            @result = cached_object do
+              @hash = serializer.attributes(options)
+              serializer.each_association do |name, association, opts|
+                if association.respond_to?(:each)
+                  array_serializer = association
+                  @hash[name] = array_serializer.map { |item| item.attributes(opts) }
                 else
-                  @result[name] = nil
+                  if association
+                    @hash[name] = association.attributes(options)
+                  else
+                    @hash[name] = nil
+                  end
                 end
               end
+              @hash
             end
           end