bat: added power_now support

Looks like current_now will eventually be deprecated. Thanks to Conrad
Calmez for finding this bug report:
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=532000

widgets/bat.lua

@@ -57,7 +57,9 @@ local function worker(format, warg)
     -- Get charge information
     if battery.current_now then
         rate = battery.current_now
-    else -- Todo: other rate sources, as with capacity?
+    elseif battery.power_now then
+        rate = battery.power_now
+    else
         return {state, percent, "N/A"}
     end