Add bench/perf.rb from 0.8 for reference

rails-api#230
rails-api#233
rails-api#242
rails-api#1305
rails-api#413
rails-api#1276

bench/perf.rb

@@ -0,0 +1,40 @@
+require 'rubygems'
+require 'bundler/setup'
+require 'rails/railtie'
+require 'active_model_serializers'
+require 'active_support/json'
+require 'benchmark'
+
+class User < Struct.new(:id, :name, :age, :about)
+  include ActiveModel::Serialization
+
+  def fast_hash
+    h = {
+      id: read_attribute_for_serialization(:id),
+      name: read_attribute_for_serialization(:name),
+      about: read_attribute_for_serialization(:about)
+    }
+    h[:age] = read_attribute_for_serialization(:age) if age > 18
+    h
+  end
+end
+
+class UserSerializer < ActiveModel::Serializer
+  attributes :id, :name, :age, :about
+
+  def include_age?
+    object.age > 18
+  end
+end
+
+u = User.new(1, 'sam', 10, 'about')
+s = UserSerializer.new(u)
+
+n = 100000
+
+Benchmark.bmbm do|x|
+  x.report('init') { n.times { UserSerializer.new(u) } }
+  x.report('fast_hash') { n.times { u.fast_hash } }
+  x.report('attributes') { n.times { UserSerializer.new(u).attributes } }
+  # x.report("serializable_hash") { n.times { u.serializable_hash } }
+end