Comment 3 for bug 534122

Implemented by calling ImageMagick "convert" from Simple Scan if it is installed. It's a bit of a hack but what it does is:
1. Use Cairo if "convert" is not in the path
2. Save all pages as JPEG and Deflate compressed TIFF images (this was required as text is not well compressed in JPEG)
3. Build a PDF using convert and the smallest of each page
4. Delete all the temporary files

The saving is a bit slow and blocks the UI while doing it but it seems to work ok.