Add discovery mode for mocked RAML document

[sichvoge]

I'd like to ask the mocking service which RAML document it currently mocks. It should use the mechanism described here.

One example of HTTP response for a hypothetical HTTP request:

curl -iv -X HEAD "http://my-api.com/v1/resources/2

...
< Link: <http://my-api.com/v1/my-api.raml>; rel="describedby" type="application/raml+yaml";

Where http://my-api.com/v1/my-api.raml should be the document including the definition for the resource http://my-api.com/{version}/resources/{id}.