Added Vietnamese translation

git-svn-id: https://plugins.svn.wordpress.org/wp-better-emails/trunk@1052435 b8457f37-d9ea-0310-8a92-e5e31aec5664
0.2.8 release:
- Fixed bug in plain text email 

git-svn-id: https://plugins.svn.wordpress.org/wp-better-emails/trunk@1052438 b8457f37-d9ea-0310-8a92-e5e31aec5664