[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/cpu.py

@@ -21,7 +21,7 @@ class Module(bumblebee.engine.Module):
         engine.input.register_callback(self, button=bumblebee.input.LEFT_MOUSE,
             cmd="gnome-system-monitor")
 
-    def utilization(self):
+    def utilization(self, widget):
         return "{:06.02f}%".format(self._utilization)
 
     def update(self, widgets):