add option to use sendmail method in gitlab.yml

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Diaspora has this option in diaspora.yml https://github.com/diaspora/diaspora/blob/develop/config/diaspora.yml.example#L649 and it is very easy to setup.

gitlab currently only has hints about using smtp method.

https://gitlab.com/gitlab-org/gitlab-ce/blob/master/config/gitlab.yml.example#L65

@marin @jacobvosmaer