[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/modules/battery.py

@@ -0,0 +1,19 @@
+import datetime
+import bumblebee.module
+
+class Module(bumblebee.module.Module):
+    def __init__(self, theme, args):
+        super(Module, self).__init__(theme, args)
+        self._battery = "BAT0" if not args else args[0]
+        self._capacity = 0
+        self._status = "Unknown"
+
+    def data(self):
+        with open("/sys/class/power_supply/%s/capacity" % self._battery) as f:
+            self._capacity = int(f.read())
+        with open("/sys/class/power_supply/%s/status" % self._battery) as f:
+            self._status = f.read().strip()
+
+        return "%02d%%" % self._capacity
+
+# vim: tabstop=8 expandtab shiftwidth=4 softtabstop=4