Mailer configured

Mailer transport is accessible throughout Locomotive context using its
`mailer` property, e.g.:

    this.mailer.sendMail({
      from: 'sender@tr.ee',
      to: 'receiver@tr.ee'
      ...
    });

On production (Heroku), we expect Mandrill to be available and its
configuration accessible via environment variables.  On development,
sendmail is used.