ext: add celery signals ep

* closes inveniosoftware/invenio-jobs#67

Author: jrcastro2

Diff for: invenio_celery/ext.py

@@ -36,8 +36,15 @@ def init_app(self, app, entry_point_group="invenio_celery.tasks", **kwargs):
         self.init_config(app)
         self.celery = FlaskCeleryExt(app).celery
         self.entry_point_group = entry_point_group
+        self.load_celery_signals()
         app.extensions["invenio-celery"] = self
 
+    def load_celery_signals(self):
+        """Load Celery signals."""
+        # Import Celery signals via entry points
+        for ep in pkg_resources.iter_entry_points("invenio_celery.signals"):
+            __import__(ep.module_name)  # Just import the module to trigger registration
+
     def load_entry_points(self):
         """Load tasks from entry points."""
         if self.entry_point_group: