[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/pulseaudio.py

@@ -42,7 +42,7 @@ class Module(bumblebee.engine.Module):
                 return line.replace(pattern, "")
         return "n/a"
 
-    def volume(self):
+    def volume(self, widget):
         if int(self._mono) > 0:
             return "{}%".format(self._mono)
         elif self._left == self._right: