add Printexc.set_uncaught_exception_handler

This patch adds the following function to the `Printexc` module:

```ocaml
val set_uncaught_exception_handler : (exn -> raw_backtrace -> unit) -> unit
```

This allow the user to specify a function to handle uncaught exceptions.

(mantis ticket: http://caml.inria.fr/mantis/view.php?id=5941)