Rename Responsive to R. Slides to avoid confusion - Responsive suggests its the only responsive option, whereas flex and nivo are also responsive

git-svn-id: https://plugins.svn.wordpress.org/ml-slider/trunk@877092 b8457f37-d9ea-0310-8a92-e5e31aec5664
Tidy up the CSS resets

git-svn-id: https://plugins.svn.wordpress.org/ml-slider/trunk@877093 b8457f37-d9ea-0310-8a92-e5e31aec5664
Tidy up WP_Image_Editor error messages

git-svn-id: https://plugins.svn.wordpress.org/ml-slider/trunk@877094 b8457f37-d9ea-0310-8a92-e5e31aec5664