[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/datetime.py

@@ -29,7 +29,7 @@ class Module(bumblebee.engine.Module):
         )
         self._fmt = self.parameter("format", default_format(self.name))
 
-    def get_time(self):
+    def get_time(self, widget):
         return datetime.datetime.now().strftime(self._fmt)
 
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