SES sending limit

SES has a sending limit of 50 see http://docs.aws.amazon.com/ses/latest/APIReference/API_SendEmail.html
therefore I needed to batch send emails with more than 50 destinators.

Also made `to` field optionnal, but you still need to provide at least 1 to,cc,bcc.
Also use a fixed version of node-ses for the previous change.
related to https://github.com/my-koop/module.communications/issues/4