user varaible doesn't exist in invite formatting code

closes #231

Author: jesopo

src/core_modules/format_activity.py

@@ -170,7 +170,7 @@ def invite(self, event):
         line = "- %s" % minimal
 
         self._event("invite", event["server"], line, event["target_channel"],
-            user=user, minimal=minimal, formatting=formatting)
+            user=event["user"], minimal=minimal, formatting=formatting)
 
     @utils.hook("received.mode.channel")
     def mode(self, event):