Category manage

# Category Manage (Admin)
* Functions:
    * Insert
    * Update
    * Manage
* Files:
    * app/Controller/CategoriesController.php
    * app/Model/Category.php
    * app/View/Categories/admin_index.ctp
    * app/View/Categories/admin_edit.ctp
Trong view tuyệt đối chỉ sử dụng Alternative syntax. Không dùng dấu ```{```