Multiple OS style support

This will allow Unity to style Atom based on the platform OS/Version. This PR adds styles for Yosemite. The theme will seamlessly update/activate based on the platform.

![screen shot 2014-09-16 at 10 10 29 pm](https://cloud.githubusercontent.com/assets/1680/4297616/d0653174-3e0f-11e4-9cdf-9bc61ad0d8a2.png)