Bug fixes and updates

* Fix double printing problem within the MailerMailer Settings page in the admin panel. The problem was due to the way wordpress registers variables. When registering new variables wordpress makes a call to 'add' (to first add the variable to the db) then 'update' (which updates the value). Both add and update make calls to the same function used to sanitize user input causing the notification to appear twice. More on this issue can be found here http://codex.wordpress.org/Function_Reference/register_setting#Notes

* Bump up plugin version for compatibility with wordpress 4.0.

* Updated version and copyright date.