Use self.* in callback to update and close watcher (nodejs#97)

onchange callback sets this to the reported FSEvent rather than the
outer FSWatcher

lib/fs.js

@@ -1380,10 +1380,10 @@ function FSWatcher() {
     // after the handle is closed, and to fire both UV_RENAME and UV_CHANGE
     // if they are set by libuv at the same time.
     if (status < 0) {
-      if (this._handle !== null) {
+      if (self._handle !== null) {
         // We don't use this.close() here to avoid firing the close event.
-        this._handle.close();
-        this._handle = null;  // make the handle garbage collectable
+        self._handle.close();
+        self._handle = null;  // make the handle garbage collectable
       }
       const error = errors.uvException({
         errno: status,