added --ds9reg option to mkrefcat.py
  * makes a ds9 region file with all sources in the catalog
Major upgrade of README.md
 * now provides detailed instructions for how to run the pipeline.
improved refimage registration
 * added a second drizzle stage for the WCS refimage construction, so that
   the refimage is WCS-aligned with the reference catalog (if provided)
 * cleaned out some unused functions from register.py