[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/disk.py

@@ -21,11 +21,13 @@ class Module(bumblebee.engine.Module):
         )
         self._path = self.parameter("path", "/")
         self._perc = 0
+        self._used = 0
+        self._size = 0
 
         engine.input.register_callback(self, button=bumblebee.input.LEFT_MOUSE,
             cmd="nautilus {}".format(self._path))
 
-    def diskspace(self):
+    def diskspace(self, widget):
         return "{} {}/{} ({:05.02f}%)".format(self._path,
             bumblebee.util.bytefmt(self._used),
             bumblebee.util.bytefmt(self._size), self._perc