use Rails for previewing emails

:warning: **WIP** :warning: 

It works!  Kinda!

Spent waaay too much time on this.  Turns out inline image previewing in Rails 4.1.5 is busted, which is being fixed here: https://github.com/rails/rails/pull/14519

Fortunately, I'm now ~20% closer to understanding wtf Engines and Railties are (unfortunately: from 0).

![screen shot 2014-09-02 at 5 45 15 am](https://cloud.githubusercontent.com/assets/86842/4117377/2c414fbc-3291-11e4-9543-24ed44f677e9.png)

Closes https://github.com/afeld/tricle/issues/64.