added transient caching to update json method

git-svn-id: https://plugins.svn.wordpress.org/nextgen-facebook/trunk@796047 b8457f37-d9ea-0310-8a92-e5e31aec5664
readme update

git-svn-id: https://plugins.svn.wordpress.org/nextgen-facebook/trunk@796048 b8457f37-d9ea-0310-8a92-e5e31aec5664