Simple magento extension to add comment field in admin order view and order grid

General information for the business: Marketplace Magento
Description of requirements/functionality: I just need an extension that :
- add a field named "admin_comment" on orders, varchar, 300 characters max
- display this field in admin order grid (admin/sales_order)
- display this field in order view page (admin/sales_order/view/order_id/xxxx/), and add a textaera to enable admin to change the value of this field

The objective of this field is to add simple admin comment or a todo like "call the customer to postpone delivery", "call the carrier to arrange delivery", ....
Specific technologies required: Magento 1.9.3
Extra notes: