Schedule the Newsletter to Automatically Send

Originally mentioned by tcibah in #673018: Implement hook_requirements(). Avoid WSOD.

In short, one should be able to schedule the newsletter to automatically send instead of manually sending it oneself.

Sunmailer currently...

• Automatically updates the cutoff date for content after it is sent so that the next newsletter will only include new content added after it was last sent
• Automatically pulls content from views instead of requiring users to manually tag content to appear in the newsletter
• Manually requires the user to send the newsletter

By adding this feature, in theory SunMailer can act completely autonomously once it has been configured and scheduled.

In addition to building the basic feature, there are some corner cases that will need to be covered:

1. Sunmailer should not send if no new content exists
2. Sunmailer should not send if it encounters unexpected errors in generating the newsletter (e.g.: the user uploads a bad XSL file, or one of the views that Sunmailer uses goes missing)

I'm deferring this until after the 6.x-1.0, but it will be quite high on my list after that. Feel free to comment on the design of this feature in the interim, though.