Add 2.3.1



git-svn-id: https://plugins.svn.wordpress.org/ecwid-shopping-cart/trunk@857147 b8457f37-d9ea-0310-8a92-e5e31aec5664
Update release 2.3.1 readme.txt stable version

git-svn-id: https://plugins.svn.wordpress.org/ecwid-shopping-cart/trunk@857154 b8457f37-d9ea-0310-8a92-e5e31aec5664