fix png files to avoid luatex compilation warning because of libpng (cf. http://tex.stackexchange.com/questions/125612/warning-pdflatex-libpng-warning-iccp-known-incorrect-srgb-profile)
make compile.sh to use the global pillar command if any. Like that I've been able to put the pillar script in my ~/bin, defined both PHARO_VM and PILLAR_IMAGE and I can directly compile any book/chapter without downloading anything in each repo