Add signals

Fixes #219

django_q/signals.py

@@ -1,7 +1,7 @@
 import importlib
 
 from django.db.models.signals import post_save
-from django.dispatch import receiver
+from django.dispatch import receiver, Signal
 from django.utils.translation import ugettext_lazy as _
 
 from django_q.conf import logger
@@ -24,3 +24,7 @@ def call_hook(sender, instance, **kwargs):
             f(instance)
         except Exception as e:
             logger.error(_('return hook {} failed on [{}] because {}').format(instance.hook, instance.name, e))
+
+
+pre_enqueue = Signal(providing_args=["task"])
+pre_execute = Signal(providing_args=["func", "task"])