Yosemite icon refresh

Replaces the Atom icon with a "Yosemite" version. Issue #3923

Current

![screen shot 2014-12-30 at 11 10 27 pm](https://cloud.githubusercontent.com/assets/378023/5579158/6837e04e-9079-11e4-91ed-dcbc9a74233a.png)

New "Yosemite" version

![screen shot 2014-12-30 at 11 11 04 pm](https://cloud.githubusercontent.com/assets/378023/5579163/7710eda4-9079-11e4-9583-47fe62ecdafc.png)

In context

![screen shot 2014-12-30 at 11 48 21 pm](https://cloud.githubusercontent.com/assets/378023/5579460/7ddec016-907e-11e4-9696-ae252f118692.png)

The new version doesn't necessarily NEED to have the white border etc. Might feel like being a stock app from Apple? /cc @cameronmcefee

- [x] Mac
- [ ] File icons
- [ ] Win
- [ ] Linux