Allow users to change the speed via shortcode

git-svn-id: https://plugins.svn.wordpress.org/rotatingtweets/trunk@875667 b8457f37-d9ea-0310-8a92-e5e31aec5664
Updating instructions to reflect new shortcode option

git-svn-id: https://plugins.svn.wordpress.org/rotatingtweets/trunk@875668 b8457f37-d9ea-0310-8a92-e5e31aec5664