Node Mailer setup

Hi !
This pull request adds a 'mailer-setup.js' file dedicated to load some custom nodemailer configurations from $ASSETS/mailer-setup.json and create a nodemailer smtp transport using those informations. Or default to classic smtpTransport if no configuration is provided.
It enables to get rid of postfix locally. I think.

Also, I added readAssetSync method to utils.