Add JSON Schema support

[Etienne]

The JSON Schema allows us to check the validity of a JSON file according to a schema. So far, I've never used it because Emacs lacks support for such things. From experience, I can tell that it's a very promising feature for using new APIs or just checking a file has a correct syntax. However, I'm also not sure if it should be implemented in json-mode or the native json library.

Of course, I don't think the validation UI should be implemented by either json-mode or json.el. We already got wonderful frameworks for that such as flycheck or flymake.
What should be implemented is a backend for validating the file (like checkdoc can do for emacs-lisp).

An other really useful thing that could be implemented after writing the checker is a JSON schema completion. This will allow us to complete what kind of format is available at a JSON path and also get documentation for it.

Again, I'm not sure if it should be implemented as an other library or included in json-mode but I think it's a good place to discuss that.