[modules] Add battery indicator plugin

Add a plugin that displays the remaining battery power in %. This also
introduces the concept of arguments that can be passed to a module
during startup by delimiting the module name with ":", for example:

-m battery:BAT1 to query the BAT1 device.

Note that this works to an arbitray length, i.e. if a module accepts 3
parameters: -m <modulename>:<A>:<B>:<C>

The module gets the arguments as list.

bumblebee/module.py

@@ -1,6 +1,6 @@
 
 class Module(object):
-    def __init__(self, theme):
+    def __init__(self, theme, args):
         self._theme = theme
 
     def theme(self):