[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/cmus.py

@@ -41,8 +41,9 @@ class Module(bumblebee.engine.Module):
         self._status = None
         self._shuffle = False
         self._repeat = False
+        self._tags = defaultdict(lambda: '')
 
-    def description(self):
+    def description(self, widget):
         return string.Formatter().vformat(self._fmt, (), self._tags)
 
     def update(self, widgets):