Better handling of operations on list options with complex types. Closes #37.

common/modules/promises.jsm

@@ -83,6 +83,13 @@ var Promises = Module("Promises", {
         });
     },
 
+    /**
+     * Returns true if the passed object is a promise.
+     */
+    isPromise: function isPromise(obj) {
+        return isObject(obj) && typeof obj.then === "function";
+    },
+
     /**
      * Returns a promise which resolves after the given number of
      * milliseconds.