Updated default icons with all required scales

According to [OS X Human Interface Guidelines][1], app icons should
provide scales of following resolutions:

* icon_512x512@2x : 1024 x 1024
* icon_512x512    : 512 x 512
* icon_256x256@2x : 512 x 512
* icon_256x256    : 256 x 256
* icon_128x128@2x : 256 x 256
* icon_128x128    : 128 x 128
* icon_32x32@2x   : 64 x 64
* icon_32x32      : 32 x 32
* icon_16x16@2x   : 32 x 32
* icon_16x16      : 16 x 16

The new icons are generated from original icon files by using `iconutil`
tool

```
iconutil -c iconset -o <file>.iconset <file>.icns
iconutil -c icns -o <file>.icns <file>.iconset
```

[1]: https://developer.apple.com/library/mac/documentation/UserExperience/Conceptual/OSXHIGuidelines/Designing.html