Put html email body in an iframe for proper rendering

The HTML part of an email is usually a full HTML document (with an `<html>` tag).  Currently letter_opener renders this document directly inside another HTML document. This results in invalid HTML. Furthermore, script tags in the inner HTML document are ignored. This breaks the `@media` queries which drive responsive email layouts.

This patch renders the HTML part in an iframe, much like [mail_view does](https://github.com/basecamp/mail_view/blob/master/lib/mail_view/email.html.erb#L99).  It will fix ryanb/letter_opener#98