add a function for not allow_url_fopen.

ref: http://in.php.net/manual/en/function.file-get-contents.php#82255

git-svn-id: http://plugins.svn.wordpress.org/featured-image-via-url/trunk@826990 b8457f37-d9ea-0310-8a92-e5e31aec5664
Merge branch 'master' of github.com:tsaiid/wp-plugin-featured-image-via-url