Adds support for meta attribute

Currently, 0.10.0.pre doesn't support `meta` option in `render`. This
way, there's no way to support features such as pagination. `0.9` had
this feature in place.

This adds support for it, as well as fixes small things in README.md.

This won't support `meta` in array responses because arrays don't have
keys, obviously. Also, the response should have a `root` key, otherwise
no `meta` will be included.

In some cases, for example using JsonApi, ArraySerializer will result in
a response with a `root`. In that case, `meta` will be included.

lib/active_model/serializer/adapter.rb

@@ -18,7 +18,8 @@ module ActiveModel
       end
 
       def as_json(options = {})
-        serializable_hash(options)
+        hash = serializable_hash(options)
+        include_meta(hash)
       end
 
       def self.create(resource, options = {})
@@ -30,6 +31,25 @@ module ActiveModel
       def self.adapter_class(adapter)
         "ActiveModel::Serializer::Adapter::#{adapter.to_s.classify}".safe_constantize
       end
+
+      private
+
+      def meta
+        serializer.meta if serializer.respond_to?(:meta)
+      end
+
+      def meta_key
+        serializer.meta_key || "meta"
+      end
+
+      def root
+        serializer.json_key
+      end
+
+      def include_meta(json)
+        json[meta_key] = meta if meta && root
+        json
+      end
     end
   end
 end