[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/output.py

@@ -41,7 +41,7 @@ class Widget(bumblebee.store.Store):
             self._full_text = value
         else:
             if callable(self._full_text):
-                return self._full_text()
+                return self._full_text(self)
             else:
                 return self._full_text