Add newsletter signup form in the plugin settings

[mindctrl]

We should add a signup form somewhere in the plugin settings so we can start building a mailing list for people using the plugin.

[mindctrl]

I'm going to punt this to 2.3 until I can wrap my head around the MailChimp form builder thing.

[pippinsplugins]

We could always just add a link to a signup form on our site.