a slew of tiny changes and fixes as per code review

git-svn-id: http://gsoc.svn.wordpress.org/2013/codebykat/post-by-email/trunk@2219 f69abe3c-2ee2-4550-9a6d-17bd458115be
properly validate settings on save

git-svn-id: http://gsoc.svn.wordpress.org/2013/codebykat/post-by-email/trunk@2220 f69abe3c-2ee2-4550-9a6d-17bd458115be