Update NSStatusItem setup to work with Yosemite's dark mode

I changed the NSStatusItem setup to use a single template image instead of multiple images. This way AppKit decides how it should be highlighted. This makes it work correctly with Yosemite's dark mode as well.
![screen shot 2014-07-08 at 5 35 10 pm](https://cloud.githubusercontent.com/assets/283886/3518446/27cbabb4-0701-11e4-91b0-ebf984ebdaea.png)

It's worth noting that this removes the third gray state for when scroll reversing is disable. I'm not sure if this is an acceptable loss for you or not.

Thanks!