Refactoring Conference/Event media to new media object

close #420

db/migrate/20140801103645_create_commercials.rb

@@ -0,0 +1,10 @@
+class CreateCommercials < ActiveRecord::Migration
+  def change
+    create_table :commercials do |t|
+      t.string :commercial_id
+      t.string :commercial_type
+      t.references :commercialable, polymorphic: true
+      t.timestamps
+    end
+  end
+end