Always return a promise from the stopper function (#296)

* feat: always return a promise from the stopper function

* Use undefined instead of null.

* Update README.md

README.md

@@ -30,13 +30,13 @@ stop(); // To end observation
 The callback passed as the second parameter to `.watch` get's called whenever the operating system detects a
 a change in the file system. It takes three arguments:
 
-###### `fsevents.watch(dirname: string, (path: string, flags: number, id: string) => void): Function`
+###### `fsevents.watch(dirname: string, (path: string, flags: number, id: string) => void): () => Promise<undefined>`
 
  * `path: string` - the item in the filesystem that have been changed
  * `flags: number` - a numeric value describing what the change was
  * `id: string` - an unique-id identifying this specific event
- 
- Returns closer callback.
+
+ Returns closer callback which when called returns a Promise resolving when the watcher process has been shut down.
 
 ###### `fsevents.getInfo(path: string, flags: number, id: string): FsEventInfo`
 

fsevents.js

@@ -24,8 +24,10 @@ function watch(path, handler) {
   let instance = Native.start(path, handler);
   if (!instance) throw new Error(`could not watch: ${path}`);
   return () => {
-    const result = instance ? Promise.resolve(instance).then(Native.stop) : null;
-    instance = null;
+    const result = instance
+        ? Promise.resolve(instance).then(Native.stop).then(() => undefined)
+        : Promise.resolve(undefined);
+    instance = undefined;
     return result;
   };
 }