Findlib support

There is some basic support for Findlib and ocamlfind. When you install ocamljs you’ll get a script ocamlfindjs which replaces ocamlfind (it’s just a wrapper). In a META file you can use the js tag for archives etc. However, ocamlfind doesn’t know anything about ocamljs; we just override the regular ocamlc, and both the byte and js tags are set, so in a META file the js entries must go before the byte entries if both are supported by the library.

I’d like to do something better here, but this works pretty well and requires no changes to Findlib.