add parentheses for ocaml operators
e.g.,
val +/ : int -> int
is now
val (+/) : int -> int
fix: add `~`, `|` and `&` as ocaml operator characters, as they were missing
(because they don't appear in the documentation http://caml.inria.fr/pub/docs/manual-ocaml-4.00/expr.html#toc51)
Merge pull request #42 from pw374/master

output parentheses for ocaml operators