support categories and tags via shortcode

git-svn-id: http://gsoc.svn.wordpress.org/2013/codebykat/post-by-email/trunk@2303 f69abe3c-2ee2-4550-9a6d-17bd458115be
PIN-based authentication

git-svn-id: http://gsoc.svn.wordpress.org/2013/codebykat/post-by-email/trunk@2304 f69abe3c-2ee2-4550-9a6d-17bd458115be