Update compatibility to 3.7.

git-svn-id: https://plugins.svn.wordpress.org/wp-user-avatar/trunk@795417 b8457f37-d9ea-0310-8a92-e5e31aec5664
Fix typo.

git-svn-id: https://plugins.svn.wordpress.org/wp-user-avatar/trunk@795430 b8457f37-d9ea-0310-8a92-e5e31aec5664
Update readme.

git-svn-id: https://plugins.svn.wordpress.org/wp-user-avatar/trunk@795452 b8457f37-d9ea-0310-8a92-e5e31aec5664