support for lwt 2.5.0, bug fixes and content-type for the REST API

• Allow raw bodies in queries and responses for the REST API. This is
  controlled by the Content-type field set by the client:
  by default, we still use JSON (or use application/json) but using
  application/octet-stream will avoid having to hex-encode large
  binary blobs to make them JSON-compatible. This feature is still
  experimental (especially when using Git on the server) (#255)
• Adapt to ocaml-git.1.7.1 (which works with lwt.2.5.0)
• Expose Store.config for all the stores (AO, RW, etc.)
• Expose Irmin_git.Internals to be able to get back the
  Git commit objects from an head value (#245, #241)
• Expose Irmin.Private.remove_node
• Remove the special __root__ filename in Irmin stores and in views
  (#233)
  • This fixes View.update_path when the view contains a value at its
    root. Now the updated path contains a the value stored at the root
    of the view.
  • Writing a value to the root of a store is now an error
  • Reading a value at the root of a store always return None
• Make the HTTP backend re-raise the Invalid_argument and Failure
  exceptions that were raised by the server.