Simplify email submit check

git-svn-id: https://plugins.svn.wordpress.org/visual-form-builder/trunk@750326 b8457f37-d9ea-0310-8a92-e5e31aec5664
Add vfb_sender_mail_header so Sender header can be controlled

git-svn-id: https://plugins.svn.wordpress.org/visual-form-builder/trunk@750328 b8457f37-d9ea-0310-8a92-e5e31aec5664
Don't add for attribute for certain form items

git-svn-id: https://plugins.svn.wordpress.org/visual-form-builder/trunk@750333 b8457f37-d9ea-0310-8a92-e5e31aec5664