Additional feartures are the followings:
The script has to check whether the user is loged in to facebook or not. If the user is not loged in the script doesn't run.
if the user is logged in the script has to check whether the user has already liked the page or not.
If the user has already liked the page the script stops and doesn't run further. If the the user hasn't liked the page yet the script runs.
The script must only allow one click, after one click the script has to stop.