add github oauth2 option

git-svn-id: https://plugins.svn.wordpress.org/wp-reposidget/trunk@1031133 b8457f37-d9ea-0310-8a92-e5e31aec5664
update readme

git-svn-id: https://plugins.svn.wordpress.org/wp-reposidget/trunk@1031139 b8457f37-d9ea-0310-8a92-e5e31aec5664