specify xy resolution (300dpi)

set xy resolution of images to 300dpi
(by default pdftoppm generates images with 150dpi resolution, which leads to poor OCR recognition)
set also resolution to 300dpi when converting image to PDF using hocr2pdf
(This ensures that layout of the text is compatible to the image dpi)
Merge pull request #3 from fritz-hh/master

Corrected dpi settings in pdftoppm to improve OCR results and in hocr2pdf to improve layout of text in final pdf