mailer: Make delivery_method configurable

delivery_method is set to smtp by default. Users can
change it to sendmail like this:

    {
      "mailer": {
        "mail_from": "sensu@example.com",
        "mail_to": "monitor@example.com",
        "delivery_method": "sendmail"
      }
    }
Merge pull request #365 from maoe/mailer-derivery-method

mailer: Make delivery_method configurable