I finally found some time to implement some new features. The work is still ongoing, but currently following new features are implemented:

• orientation modes (automatic, portrait, landscape)
  • automatic mode uses orientation sensor to rotate the image, portrait and landscape will lock the orientation
• Cropping possible after image has been taken/selected
  • Actually this performs a perspective correction & crop for a given quadrilateral:

I'll probably add support for text extraction from PDF files too. Some small enchancements for example to camera behaviour will be done too.