Retrieve docs

Closes #3 

Hey @devcritter if you are able to, a review would be good.
I'm afraid this isn't too clear to me (I probably should've mention something when I saw this method in the content handler review!) - but so will the XDS interface request specific handlers, such as the CDA handler? It just seems to break the SHR design a bit if an interface module *knows* what handlers need to get used. Not to imply that it shouldn't, but if that's the case then there's probably something wrong in our design.