Add support for preview PNG and PDF download

[smarr]

Do something along the lines of:

convert pgas.pdf  pgas_%04d.png
montage -background #ffffff -geometry +0+0 -tile 10x4 pgas_*.png montage_pgas.png

This will create a PNG file giving a preview of all pages.
Inspiration for this was: http://youtu.be/hNENiG7LAnc