switching mail to use actionmailer, using letter opener web for previewing

If we use ActionMailer, then we can take advantage of some neat features and gems.  For instance, Sidekiq has built in ActionMailer functionality

https://github.com/mperham/sidekiq/wiki/Delayed-Extensions

There is also an nice email preview app

https://github.com/fgrehm/letter_opener_web based on https://github.com/ryanb/letter_opener

This PR addresses #3, but still needs some work, but I was hoping you could take a look and let me know if you liked the direction. The ability to change SMTP settings for different campaigns was not as simple to implement as I had thought, so I need to make sure I didn't break anything in the process, and clean up the code a little.

I'm also a little confused on what the templates are supposed to look like with the attachments included.  ActionMailer supports attachments, however, so we should be able to make it work.