Check for PHP's allow_url_fopen ini variable

To quote the person who activated my CVS account, AjK:

It would be a sensible idea to check PHP's allow_url_fopen ini variable [1] when the module is enabled so if it's disallowed you can warn the end user that this module won't work. Many shared hosting providers switch this off for security reasons. (This stems from your use of file_get_contents(url) to fetch data from a remote server using URL wrappers).