ENH: added preprocessing script

PCampReviewPreprocessor will organize DICOM data into a directory hierarchy
recognized by PCampReview.

TODO:
 * clean up
 * remove dependency on the local dcm2xml (the binary takes > 1M on Mac, need
    to discuss)
 * consider (again) PCampReview work directly with DICOM database; the
    disadvantage is that NRRD conversion has to happen anyway for the statistics
    calculation, although it is not part of PCampReview workflow at the moment
 * make this run from the command line and add instructions