Allow fetching json response from Flickr (for a few methods)

Enable to fetch json-response from flickr (no unmarshalling).

Would be feasible for my app. I've done a 1st attempt in a way which will not break existing API, however, it is not very elegant.

The main change would be Transport#getJson.
Then i've added PhotosInterface#getInfoAsJson and getSizesAsJson (as alternative to getInfo) ... and i would need more.
(For this reason i've also introduced JSONResponse and JSONResponseImpl.)

I could add all the methods i need, but to do it for at least all get#methods on PhotosInterface and PhotosetsInterface it would be a lot of boilerplate work duplicating all the methods ... not a good approach.
So would be interested in how to enable json in a more elegant way (and to avoid to break API  ... changing the signature of these getter methods).