Strip all png images to avoid libpng warning

With the new libpng, Slic3r produces a warning dialog on each start saying: *iCCP: Not recognized known sRGB profile that has been edited*

To avoid this, I've stripped all pngs in var directory and the warning is gone. I was inspired with this relevant thread about LaTeX: http://tex.stackexchange.com/questions/125612/warning-pdflatex-libpng-warning-iccp-known-incorrect-srgb-profile