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.

test/serializers/meta_test.rb

@@ -0,0 +1,77 @@
+require 'test_helper'
+
+module ActiveModel
+  class Serializer
+    class MetaTest < Minitest::Test
+      def setup
+        @blog = Blog.new(id: 1,
+                         name: 'AMS Hints',
+                         writer: Author.new(id: 2, name: "Steve"),
+                         articles: [Post.new(id: 3, title: "AMS")])
+      end
+
+      def test_meta_is_present_with_root
+        adapter = load_adapter(root: "blog", meta: {total: 10})
+        expected = {
+          "blog" => {
+            id: 1,
+            title: "AMS Hints"
+          },
+          "meta" => {
+            total: 10
+          }
+        }
+        assert_equal expected, adapter.as_json
+      end
+
+      def test_meta_is_not_included_when_root_is_missing
+        adapter = load_adapter(meta: {total: 10})
+        expected = {
+          id: 1,
+          title: "AMS Hints"
+        }
+        assert_equal expected, adapter.as_json
+      end
+
+      def test_meta_key_is_used
+        adapter = load_adapter(root: "blog", meta: {total: 10}, meta_key: "haha_meta")
+        expected = {
+          "blog" => {
+            id: 1,
+            title: "AMS Hints"
+          },
+          "haha_meta" => {
+            total: 10
+          }
+        }
+        assert_equal expected, adapter.as_json
+      end
+
+      def test_meta_is_not_used_on_arrays
+        serializer = ArraySerializer.new([@blog], root: "blog", meta: {total: 10}, meta_key: "haha_meta")
+        adapter = ActiveModel::Serializer::Adapter::Json.new(serializer)
+        expected = [{
+          id: 1,
+          name: "AMS Hints",
+          writer: {
+            id: 2,
+            name: "Steve"
+          },
+          articles: [{
+            id: 3,
+            title: "AMS",
+            body: nil
+          }]
+        }]
+        assert_equal expected, adapter.as_json
+      end
+
+      private
+
+      def load_adapter(options)
+        serializer = AlternateBlogSerializer.new(@blog, options)
+        ActiveModel::Serializer::Adapter::Json.new(serializer)
+      end
+    end
+  end
+end