updated support for avatars with default value

git-svn-id: https://plugins.svn.wordpress.org/m-retina-gravatars/trunk@821972 b8457f37-d9ea-0310-8a92-e5e31aec5664
updated stable tag in readme

git-svn-id: https://plugins.svn.wordpress.org/m-retina-gravatars/trunk@821999 b8457f37-d9ea-0310-8a92-e5e31aec5664