[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/test.py

@@ -10,7 +10,4 @@ class Module(bumblebee.engine.Module):
             bumblebee.output.Widget(full_text="test")
         )
 
-    def update(self, widgets):
-        pass
-
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