FAQ, Installation sections added, other sections cleaned up and validated with WordPress ReadMe.txt validator.

git-svn-id: https://plugins.svn.wordpress.org/email-users/trunk@799230 b8457f37-d9ea-0310-8a92-e5e31aec5664
Add integration support for ItThinx Groups plugin.

git-svn-id: https://plugins.svn.wordpress.org/email-users/trunk@799231 b8457f37-d9ea-0310-8a92-e5e31aec5664