Library: change Js.wrap_meth_callback to allow methods with no parameter.

Also add unsafe variants of Js.wrap_meth_callback and Js.wrap_callback
that do not deal with partial and over application. This is used for
event callbacks, that take no argument in Internet Explorer and take the
event as argument in all other browsers.

Conflicts:
	runtime/jslib_js_of_ocaml.js