Fixed some bugs in tasks, apple feature:

* Update ordergroup stats when task is destroyed.
* Removed assigned caching attribute in task object.
* A lot of eager loading for tasks controller.

db/migrate/20121216180646_remove_assigned_from_tasks.rb

@@ -0,0 +1,9 @@
+class RemoveAssignedFromTasks < ActiveRecord::Migration
+  def up
+    remove_column :tasks, :assigned
+  end
+
+  def down
+    add_column :tasks, :assigned, :boolean
+  end
+end