Added option to Base64 encode email.

git-svn-id: https://plugins.svn.wordpress.org/email-users/trunk@989221 b8457f37-d9ea-0310-8a92-e5e31aec5664
Prep for 4.6.10 release.

git-svn-id: https://plugins.svn.wordpress.org/email-users/trunk@989222 b8457f37-d9ea-0310-8a92-e5e31aec5664
Updated language translation files with new strings.

git-svn-id: https://plugins.svn.wordpress.org/email-users/trunk@989223 b8457f37-d9ea-0310-8a92-e5e31aec5664