Add email notification.

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 all failing builds
    # all_broken_builds: true

    # Add committer to recipients list
    # add_committer: 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.

The email settings can be managed in the project settings dialog.

Screenshot:

![screen shot 2013-11-30 at 11 20 14 pm](https://f.cloud.github.com/assets/10019/1649553/af51da28-5a0d-11e3-9eaf-0928d84f5fe1.png)