Simplenews & Massmailer Integration

Issue Summary

Just took a look at the README, took a quick look at the function _newsletter_checkbox_form_submit(). I was going to modify the simplenews_block_form and try to replace the function you provided for the Campaign Mailer. Just don't have the time right now to do this. Would be nice if there was a simple way to add this functionality to one of the native newsletter applications however. It's a good concept!