Added up and down migration for weekly task data.

db/migrate/20130615073715_create_periodic_task_groups.rb

@@ -0,0 +1,13 @@
+class CreatePeriodicTaskGroups < ActiveRecord::Migration
+  def change
+    create_table :periodic_task_groups do |t|
+      t.date :next_task_date
+
+      t.timestamps
+    end
+
+    change_table :tasks do |t|
+      t.references :periodic_task_group
+    end
+  end
+end