Growing text area in footer

I am trying to achieve functionality similar to writing comments in facebook app.

I would like to place attached to bottom a textarea that can grow in height as user types and a post button along side.
I can place text area in footer but footer restricts textarea’s growth as user types more than one line.
OR I place it in content and somehow force this to bottom.

How to go about it?

I think this is what you’re looking for: