WIP: Get power level with gnome 3.8 on ubuntu 14.04

Not based on any docs, discovered by poking at looking glass console.
`GetPrimaryDeviceSync()[0]` returns:

    ["/org/freedesktop/UPower/devices/battery_BAT0", 
     2, 
     ". GThemedIcon battery-good-symbolic gpm-battery-060 battery-good",
     50,  /* this appears to be my power level */
     2, 
     3961]

Surely other stuff won't work.
`_proxy.IsPresent` doesn't exist.  `GetPrimaryDeviceSync()` throws exception if AC is plugged.
(BTW, I don't see why I shouldn't see the level when charging!)