closures and js_of_ocaml

[Armael]

Running ocaml-memgraph under js_of_ocaml means that the closure representation is different from native and bytecode.
For instance, trying to display the graph for (fun () () -> ()) will raise an assertion failure in mk_closinfo.

One idea would be to read Sys.backend_type to avoid reading into a closure block in the Other case.

[Armael]

Fixed by #10 .