Added IP check for security

The API documentation says that scripts should include a check to ensure the GET request was made by the IP that blockchain.info points to. I have added in this check to prevent people faking callbacks. 
Appending another GET variable to your query string as a secret would also be advisable to further ensure validity. For example you could make your callback URL http://exampl.com/callback.php?secret=abc123 and then check that $_GET['secret'] is correctly set in your script.