Add support for making API paths relative to the specified api_path setting

[MrTam]

I ran into an issue with DRS, when coupled with the excellent swagger-codegen project.

My URI scheme prefixes the API, in the format of /api/v1/foo. Generated URIs retrieved from the DRF routers fully qualify ViewSets with this prefix.

With the api_path set to default (/), full paths appear in the generated docs. This means that swagger-codegen parses the name incorrectly, as it always expects resources to be relative to their base path.

Also, if i do set the api_path to be /api/v1/foo, generated client libraries try to access the resource at /api/v1/foo/api/v1/foo/resource. I think this behaviour is incorrect, as the APIs are supposed to be relative to the resource basePath.

This PR adds support for this functionality. With the new relative_paths setting set to True, URIs shall be relative to the api_path setting. Default value is False, to preserve existing behaviour.