OS X: adding missing sizes in the .icns files

E.g.:
![icon_16x16](https://cloud.githubusercontent.com/assets/3859425/5327656/9d121b2c-7d5a-11e4-9d00-1074e41cc923.png)
![icon_16x16 2x](https://cloud.githubusercontent.com/assets/3859425/5327657/9d1295b6-7d5a-11e4-8d68-5b2fdc366df1.png)
![icon_32x32](https://cloud.githubusercontent.com/assets/3859425/5327655/9d101b1a-7d5a-11e4-99de-f62cd832f8da.png)
![icon_32x32 2x](https://cloud.githubusercontent.com/assets/3859425/5327659/9d14d86c-7d5a-11e4-8f82-c76bf8ec40e7.png)
![icon_128x128](https://cloud.githubusercontent.com/assets/3859425/5327660/9d1793fe-7d5a-11e4-9229-e83a1f27b5a3.png)
![icon_128x128 2x](https://cloud.githubusercontent.com/assets/3859425/5327658/9d13e4f2-7d5a-11e4-8ebd-69c67d11b6ee.png)
![icon_256x256](https://cloud.githubusercontent.com/assets/3859425/5327661/9d29092c-7d5a-11e4-817b-d5dd2750af44.png)
![icon_256x256 2x](https://cloud.githubusercontent.com/assets/3859425/5327662/9d32ad60-7d5a-11e4-8289-1e7096118594.png)
![icon_512x512](https://cloud.githubusercontent.com/assets/3859425/5327664/9d335dd2-7d5a-11e4-8d96-85bde6503741.png)
![icon_512x512 2x](https://cloud.githubusercontent.com/assets/3859425/5327663/9d334108-7d5a-11e4-88af-d581d6b8063e.png)