Different status codes in the API lib are returned on hook creation, update or deletion.
If a required parameter is not given (e.g. `url` in `/projects/:id/hooks/:hook_id`) status
code 400 (Bad request) is returned. On hook deletion a 200 status code is returned, regardless if
the hook is present or not. This makes the DELETE function an idempotent operation. Appropriate tests
are added to check these status codes.

When adding a project hook a url must be specified or a 400 error code is returned

* Specs added to check status code on handling project hooks
* refactored code, extracted a method

When a user is added to a project that is already a member of, a status code 201 is now returned to
signal an idempotent operation. If something fails then instead of returning error code 404 different
more specific error codes are returned. Status code 400 (Bad request) is returned when a required
attribute, e.g. `access_level` is not given or 422 if there is a semantic error, e.g. should
the `access_level` have an unsupported value.

Specs are added to check these status codes.

fix the gitlab-shell clone url in master installation doc

A user must be part of the team to see a protected project. A test is given
to check that a 404 error is returned if the user can not see the project.