Added support for %POST_AUTHOR% keyword replacement.

git-svn-id: https://plugins.svn.wordpress.org/email-users/trunk@765987 b8457f37-d9ea-0310-8a92-e5e31aec5664
Prep for v4.5.1 release.

git-svn-id: https://plugins.svn.wordpress.org/email-users/trunk@765988 b8457f37-d9ea-0310-8a92-e5e31aec5664