Updated the relative links in ## Contents to point to the correct documents.

 * Closing #3066
 * Closing #3609

All the info to return codes from the API functions are available in the `README.md` file as suggested.

The documentation was unclear about how to pass the api private token as
header. Updated the doc to include the nessesary information bits and
added an example using the curl command line client.

The API version is currently not equal to the gitlab major version number. Gitlab 4.1 
still uses API version 3. Point to the lib/api.rb file instead which contains the
autoritative information.

Merge branch 'master' into non-interactive-aws-install

Conflicts:
	doc/installation.md

Fix merge mess in installation.md

Also removes `closed` attribute option from create action.