Optimize images using lossless compression

I've optimized the PNG images using Zopfli. Quality has not been compromised.

 assets/ico/apple-touch-icon-114-precomposed.png | Bin 4881 -> 2939 bytes
 assets/ico/apple-touch-icon-144-precomposed.png | Bin 5659 -> 3508 bytes
 assets/ico/apple-touch-icon-57-precomposed.png  | Bin 2939 -> 1577 bytes
 assets/ico/apple-touch-icon-72-precomposed.png  | Bin 3468 -> 1932 bytes
 assets/ico/favicon.png                          | Bin 1919 -> 889 bytes