Add a loopWhile primitive to Asyc module

src/UnitJS/Async.js

@@ -3,6 +3,7 @@ return {
 	bind: bind,
 	fail: fail,
 	http: http,
+	loopWhile: loopWhile,
 	map: map,
 	parallel: parallel,
 	run: run,
@@ -100,6 +101,16 @@ function sequence() {
 	};
 }
 
+function loopWhile(predicate, body) {
+	return bind(predicate, function(keepOn) {
+		if(keepOn) {
+			return sequence(body, loopWhile(predicate, body));
+		} else {
+			return wrap();
+		}
+	});
+}
+
 function wait(delay) {
 	return function(f) {
 		setTimeout(f, delay);