Add plist key for high-res window chrome on OS X

Hey Shane, this is a start for the retina Mac stuff. I couldn't find any contributing guidelines for the project, so I hope this OK. This'll make the window support retina stuff and make the titlebar automatically retina-res.

Here's before:

![before](https://f.cloud.github.com/assets/6129/365507/a544f48c-a24a-11e2-9a92-5994076ff74e.png)

And after:

![after](https://f.cloud.github.com/assets/6129/365508/ab19016e-a24a-11e2-80e7-143f34878c2d.png)