Improvement: Added an empty option to the Lists dropdown to better convey when one isn't selected. Refactor: Set up the EDD_MailPoet class as a singleton so that actions can be unhooked. Refactor: Change plugin author name and URI.
New: Added an option to set the default checked status of the opt-in checkbox on checkout
New: Added compatibility with MailPoet 3 Tweak: Added sub-section for MailPoet settings in the extensions tab
