[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/battery.py

@@ -24,7 +24,7 @@ class Module(bumblebee.engine.Module):
         self._capacity = 100
         self._ac = False
 
-    def capacity(self):
+    def capacity(self, widget):
         if self._ac:
             return "ac"
         if self._capacity == -1: