New status icon, with Retina support

The previous one looks grainy on a Retina monitor, and it has white edges when the menu bar is semi-transparent.

I also took the liberty of changing the highlighted state to a plain white one, because it's more consistent with OS X.

Screens:

![](http://i.imgur.com/G9PzC3J.png)
![](http://i.imgur.com/pp0d7Op.png)