1. convert4kindle.sh : scale slide for kindle screen by imagemagick.
step-0 : check suffix if file is not pdf format. convert it to pdf by unoconv
step-1 : convert pdf to png
step-2 : resize png let it fit in with kindle screen
step-3 : combination

2. over_crop.py : another scale tools for slide by pyPdf
it is purposed to scale-out-crop for the kindle resolution by Pdf. but the kindle 
3 is too smart that will auto-cutting white margin. it is not only scale-out-crop 
for kindle but alos addition two samll circle at lowleft and upperrigh to spin its 
margin.