added mail_helper.js for local email testing

mail_helper.js is a small SMTP server for dev and unit test use. It has an HTTP endpoint for retrieving the verification code for an email address, and it logs the verification url and a curl command to verify the code.

`npm start` will now start a `mail_helper` along with `key_server`. Its not ideal, but you can find the log lines in the output by searching for `curl`. One improvement could be to use tmux and have a tab for each process. Whatevs.

Verification tests should work with or without a running `npm start`

Fixes #317 and #120

cc @zaach @warner this goes along with #370

r? @rfk @chilts