Implement jsoo constructor deriver.

[Drup]

Self reminder for the second part of https://github.com/ocsigen/js_of_ocaml/pull/239#issuecomment-83037593. Here was the description:

• A deriver would provide the ability to define "object constructors":

type t = < foo : int prop ; bar : int Js.optdef prop > Js.t
[@@deriving jsoo.constructor]

would define a function of type:

?bar:int -> ~foo:int -> unit -> t 

[hhugo]

could be used to implement #801 maybe ..