Switched to a new App Icon

I've designed a new icon based on Yosemite's system set.  

I updated and renamed the sized PNGs, and updated Contents.json to correspond to the new icon files.

![commit_1](https://cloud.githubusercontent.com/assets/8173499/3593514/ab3adde0-0c87-11e4-842c-f5f32ac1e9bf.png)