Change Email Attachment Behavior to wrap mail parts in multipart/related

This is the same work in #1403 by @limansky rebased on top of the current master branch. Going to do some testing here to ensure that email behavior works as expected across various email clients. If this works, then it closes #1197 and we'll have working Outlookyness.

I may take the opportunity to do some spec addition and variable name cleanup as well.