Added support for new CC Sender option, POST_CONTENT keyword, and cleaned up email header construction.

git-svn-id: https://plugins.svn.wordpress.org/email-users/trunk@774025 b8457f37-d9ea-0310-8a92-e5e31aec5664
Prep for v4.5.3 release.

git-svn-id: https://plugins.svn.wordpress.org/email-users/trunk@774026 b8457f37-d9ea-0310-8a92-e5e31aec5664