Add configuration for sendmail

If `SENDMAIL_ENABLED` is set to `true` then postfix will be started and Gitlab will be configured to use sendmail. If both `SENDMAIL_ENABLED` and `SMTP_ENABLED` are `true` then the sendmail feature is disabled. The configuration for postfix is simple; it is setup as a relay.

Postfix forks into the background and as a result cannot be started under supervisord. See the following link for more background information:

http://goo.gl/F4neAQ

Closes sameersbn/docker-gitlab#208

I opted to use postfix over sendmail because postfix is the default MTA on Ubuntu and is significantly easier to configure.