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_api.rb

@@ -23,10 +23,12 @@ module ActiveModel
               self.class.new(s, @options.merge(top: @top, fieldset: @fieldset)).serializable_hash[@root]
             end
           else
-            @hash[@root] = attributes_for_serializer(serializer, @options)
-            add_resource_links(@hash[@root], serializer)
+            @hash = cached_object do
+              @hash[@root] = attributes_for_serializer(serializer, @options)
+              add_resource_links(@hash[@root], serializer)
+              @hash
+            end
           end
-
           @hash
         end