Add email notification to committer.

I've ported the email implementaion of GitlabHQ to this repo.

Default only failing builds trigger an email notification, but this can be changed in *application.yml*

    # Email address used in the "From" field in mails sent by GitLab-CI
    email_from: gitlab-ci@localhost
  
    # Email address of your support contact (default: same as email_from)
    support_email: support@localhost
  
    # Only send emails for failing builds
    # only_fail_notifications: true

Updated instructions in installation.md how to setup smtp settings and 
I've added specs, but I had troubles running the project locally, so I couldn't test it on actual builds.

Next steps are to be able to specify the settings of each project individual and to make it possible to add other recipients in the projects settings, but I think this commit is already valuable.