sort.py & main.py
   - better construction of the epoch list file, in which we print
    the list of exposures, sorted by epoch/filter/visit.
      This now has a usefully formatted summary of the exposures available.
   - replaced --dosort with --dosetup as the option that defines whether
     we make epoch subdirs and copy in the flt files
renamed main.py to sndrizzle.py

sndrizzle.py :
    - adjusted the WCS refimage construction so that it is constructed in
    its own directory <outroot>.refim
    - rearranged the order of the processing stages, so the wcs refim is
      made before any first-pass astrodrizzling is done.

minor changes in output messages, and cleaned up some code style.