[OCaml] Privacy type
[OCaml] Interface for error module
[OCaml] Apply new coding style, make the code shorter and smarter

Coding style:
- We don't need signatures in the .ml, looking at the .mli is enought
- Signatures are still in the .ml if the value is not exported (and not trivial), but in the code, not in comments
- Usage of the ocamldoc syntax (** comment *) in the interfaces for title and functions behavior
- Use more interfaces .mli to avoid exporting urelevant values

Make the code shorter and smarter:
- "Shortcut" functions to handle a whole request
- Api.RequestType module (GET, POST, ...)
- Curl parameters propagated (RequestType + auth)