A Few Updates

We've been using this plugin and had a couple hiccups.  I've refactored to remove debug notices, updated the readme to say it's compatible with 3.9 and to match the one you get from the wp.org repo, added a boolean false check on the check for the html header so that if someone passes `'text/html'` as a header it still sends as html and added a nonce check to the admin section.  Let me know if you see any issues.