[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

tests/modules/test_modules.py

@@ -35,6 +35,7 @@ class TestGenericModules(unittest.TestCase):
                 assertWidgetAttributes(self, widget)
                 widget.set("variable", "value")
                 self.assertEquals(widget.get("variable", None), "value")
+                self.assertTrue(isinstance(widget.full_text(), str))
 
     @mock.patch("subprocess.Popen")
     def test_update(self, mock_output):