fixed issue where images would be regenerated even if present

git-svn-id: https://plugins.svn.wordpress.org/optimize-images-resizing/trunk@994645 b8457f37-d9ea-0310-8a92-e5e31aec5664
updated readme and plugin version

git-svn-id: https://plugins.svn.wordpress.org/optimize-images-resizing/trunk@994647 b8457f37-d9ea-0310-8a92-e5e31aec5664