Add raw file endpoint to API

• Zendesk ticket: https://gitlab.zendesk.com/agent/tickets/13340
• Dev link: None
• GitLab version: 8.2

It would be nice to have an API endpoint that will return a blog containing file contents. We currently have the ability to GET a file via http://docs.gitlab.com/ee/api/repository_files.html. However, this returns JSON content with a bunch of other fields. Can we add a 'raw' endpoint that just returns the content and nothing else?

Next steps

cc/ @JobV @dzaporozhets Is this something we think is useful?