fix issue "Cannot redeclare class TwitterOAuth in (...)/wp-content/plugins/pay-with-a-tweet/lib/twitteroauth.php on line 15"

git-svn-id: https://plugins.svn.wordpress.org/pay-with-a-tweet/trunk@884158 b8457f37-d9ea-0310-8a92-e5e31aec5664