* preprocessing/deskew.hh,
	* preprocessing/deskew_crop.hh: New.

	* preprocessing/unskew.hh: Remove. Deprecated.

	* tests/preprocessing/unskew.cc: Rename as...
	* tests/preprocessing/deskew.cc: ... this.

	* tests/preprocessing/Makefile.am: Update.

	* tests/img/text_to_group.pgm: New test image.