optimize png images using zopflipng

[PeterDaveHello]

Use Google's zopflipng to re-compress the png images losslessly as below:

 docs/public/images/gray.png        | Bin 56906 -> 56546 bytes
 img/dropdown-arrow.png             | Bin 229 -> 170 bytes
 img/glyphicons-blackboard.png      | Bin 3779 -> 2902 bytes
 img/glyphicons-halflings-white.png | Bin 8777 -> 8622 bytes
 img/glyphicons-halflings.png       | Bin 12799 -> 12586 bytes
 img/glyphicons-log-in.png          | Bin 3793 -> 2874 bytes
 img/glyphicons-log-out.png         | Bin 3788 -> 2875 bytes