Added config.ru

While we don't use Rack to serve our documentation publicly, being
Rack-compatible is useful for local development (such as with
http://pow.cx).  This includes the necessary voodoo to make
middleman play nicely with Rack.

config.ru

@@ -0,0 +1,19 @@
+require "rubygems"
+require "bundler/setup"
+require "middleman-core/load_paths"
+
+Middleman.setup_load_paths
+
+require "middleman-core"
+require "middleman-core/preview_server"
+
+module Middleman::PreviewServer
+  def self.preview_in_rack
+    @options = {  }
+    @app = new_app
+    start_file_watcher
+  end
+end
+
+Middleman::PreviewServer.preview_in_rack
+run Middleman::PreviewServer.app.class.to_rack_app