Stop all tasks on shutdown

This stops all of the working jobs when turbo-hipster is closing down

Change-Id: Ic160f75bd8222b233f81d269e4c035f95be11e47
diff --git a/turbo_hipster/lib/models.py b/turbo_hipster/lib/models.py
index d445379..be3f1d9 100644
--- a/turbo_hipster/lib/models.py
+++ b/turbo_hipster/lib/models.py
@@ -68,11 +68,11 @@
                 if not self.cancelled:
                     self.job.sendWorkException(str(e).encode('utf-8'))
 
-    def stop_worker(self, number):
-        # Check the number is for this job instance
+    def stop_working(self, number=None):
+        # Check the number is for this job instance (None will cancel all)
         # (makes it possible to run multiple workers with this task
         # on this server)
-        if number == self.job.unique:
+        if number is None or number == self.job.unique:
             self.log.debug("We've been asked to stop by our gearman manager")
             self.cancelled = True
             # TODO: Work out how to kill current step