[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/spacer.py

@@ -17,10 +17,7 @@ class Module(bumblebee.engine.Module):
         )
         self._text = self.parameter("text", "")
 
-    def text(self):
+    def text(self, widget):
         return self._text
 
-    def update(self, widgets):
-        pass
-
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