Replace sendmail with direct in nodemail

Fixes #1538.
* upgraded nodemailer to 0.5.15 (current master as of commit)
* implemented direct mail sending, and ripped out sendmail (we don't need sendmail anymore with direct being in place)
* modified tests to look for direct instead of sendmail
* implemented directmail response data
* made `send` function more promising
* added `successmessage` to successful mail notification function
* added rudimentary error handling for rejected / failed email messages