Added a default avatar image for users without one

usercache.py

@@ -99,6 +99,11 @@ def add_to_av_cache(user):
             loader.write(str(data))
             image = loader.get_pixbuf()
             loader.close()
+
+            if image is None:
+                image = gtk.gdk.pixbuf_new_from_file('ui/question_mark.svg')
+            image = image.scale_simple(48, 48, gtk.gdk.INTERP_BILINEAR)
+
             with AvCache().lock:
                 AvCache().map[user.screen_name] = image
         except (URLError, ValueError):