Use a default for gitlab_url that usually works
Mention Unicorn on a domain socket as an exception
Merge branch 'working_gitlab_url' into 'master'

Use a default for gitlab_url that usually works

This default lets gitlab-shell connect to Unicorn directly, instead of
going through Nginx. This has the following benefits:

- the majority of users no longer has to edit the `gitlab_url`;
- avoid issues when the external GitLab web address does not resolve on
  the GitLab server itself;
- avoid SSL certificate issues.

See merge request !32
Merge branch 'master' of github.com:gitlabhq/gitlab-shell