[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/caffeine.py

@@ -16,7 +16,7 @@ class Module(bumblebee.engine.Module):
             cmd=self._toggle
         )
 
-    def caffeine(self):
+    def caffeine(self, widget):
         return ""
 
     def state(self, widget):
@@ -33,9 +33,6 @@ class Module(bumblebee.engine.Module):
                 return False
         return False
 
-    def update(self, widgets):
-        pass
-
     def _toggle(self, widget):
         if self._active():
             bumblebee.util.execute("xset s default")