new module to make reference catalogs; better tweakreg parameter handling
 * mkrefcat.py : convert a catalog (e.g. from sextractor/tfit) into the 2 or
   3-column format for use with tweakreg.  This module is executable from
   the command line.
 * register.py / sndrizpipe.py : renamed user parameters --peakmin / --peakmax
   to --fluxmin / --fluxmax to match the tweakreg parameter names.
Added --refnbright option.
 * the refnbright option gets passed straight on to TweakReg.
  and sets the number of bright sources from the reference catalog that
  get used for catalog matching.
 * also: switched the tweakreg reference catalog from using 'mag' for
  the flux column units, to assuming 'flux'.  That is, bigger numbers
  in the flux column of the refcat are now assumed to be brighter
  sources.