[modules/ping] Re-enable ping module

Show RTT measured by ICMP echo request/replies for a given host.

For that to work correctly, change the "full_text" callback for a widget
so that the widget itself is also passed as argument in the callback
method. That actually makes a lot of sense, since the widget can now be
used as a repository of state information.

see #23

bumblebee/modules/brightness.py

@@ -24,7 +24,7 @@ class Module(bumblebee.engine.Module):
         engine.input.register_callback(self, button=bumblebee.input.WHEEL_DOWN,
             cmd="xbacklight -{}%".format(step))
 
-    def brightness(self):
+    def brightness(self, widget):
         return "{:03.0f}%".format(self._brightness)
 
     def update(self, widgets):