added new screenshots, updated readme.txt to reflect that

git-svn-id: https://plugins.svn.wordpress.org/svg-vector-icon-plugin/trunk@751620 b8457f37-d9ea-0310-8a92-e5e31aec5664
remove screenshot-4.jpg

git-svn-id: https://plugins.svn.wordpress.org/svg-vector-icon-plugin/trunk@751621 b8457f37-d9ea-0310-8a92-e5e31aec5664