created item groups and link banners and their associations and fixed bugs with join tables

front-api/db/migrate/20150322044941_create_item_groups.rb

@@ -0,0 +1,13 @@
+class CreateItemGroups < ActiveRecord::Migration
+  def change
+    create_table :item_groups do |t|
+      t.string :name
+      t.text :description
+      t.boolean :visible
+
+      t.timestamps null: false
+    end
+  end
+
+  create_join_table :item, :item_groups
+end