Add railtie to set the page_cache_directory by default

Fixes #5

Author: Rafael Mendonça França

lib/actionpack/page_caching.rb

@@ -1 +1,2 @@
 require 'action_controller/page_caching'
+require 'actionpack/page_caching/railtie'

lib/actionpack/page_caching/railtie.rb

@@ -0,0 +1,11 @@
+require 'rails/railtie'
+
+module ActionPack
+  module PageCaching
+    class Railtie < Rails::Railtie
+      initializer 'action_pack.page_caching.set_config', before: 'action_controller.set_configs' do |app|
+        app.config.action_controller.page_cache_directory ||= app.config.paths['public'].first
+      end
+    end
+  end
+end