set the default GITLAB_PORT to 443 if https is enabled, else set it to 80
construct the gitlab_url based on whether https is enabled and whether the default ports are changed
use https protocol in the wget request if https is enabled
fix git over ssh when the default http/https ports are not used.

Internally the SSH and the http/https servers always bind to ports 22 and
80/443 respectively. If the GITLAB_PORT variable is set to any other
port, then cloning over SSH would not work.

This is because internally gitlab-shell tries to communicate with the
gitlab server over http interface. In gitlab-shell configured with
the GITLAB_PORT value the conmmunication will effective fail and thereby
the cloning operation would return an error.

To fix this, we make gitlab-shell communicate with the http server
directly with the default http/https ports since this is all internal.

Prior to this update, cloning repositories over ssh protocol would not
work when the container is started in the following way:

```bash
docker run -i -t --rm \
  -p 8888:80 -p 2222:22 \
  -e "GITLAB_PORT=8888" -e "GITLAB_SHELL_SSH_PORT=2222" \
  sameersbn/gitlab
```
Merge branch 'feature/fix-usage-with-non-standard-ports', fixes #53, #34