Add basic server status page at /status.

Lists versions of various dependencies plus some Postgres and Redis
metrics.

app/controllers/status_controller.rb

@@ -0,0 +1,8 @@
+class StatusController < ApplicationController
+  respond_to :html, :json, :xml
+
+  def show
+    @status = ServerStatus.new
+    respond_with(@status)
+  end
+end