Optimize output pngs

[fredszaq]

This action should be triggered only in certain cases (ie a release build) as it will certainly take some time, and it is complicated to know without opening the file if it can be optimized.

Here is an interesting article about png optimization :
http://optipng.sourceforge.net/pngtech/optipng.html

Available optimizers we could use :

• optipng (external program)
• pngtastic, (developed in java with an MIT licence)