use tiff2pdf instead of converting first to ps and then to pdf

this is not only a lot faster, but also allows lossless conversion.
Along that line, don't try to alter the pagesize during conversion to
PDF. PDF-viewer applications do a great job adjusting to the paper-size
as the user wants it, so let the user make the decision. After all when
having the PDF, it is unlikely that it will be printed out anyway.
Merge pull request #2 from cloph/pr_tiff2pdf

use tiff2pdf instead of converting first to ps and then to pdf