CR rejection bugfix; improved help formatting.
 * the drizzle parameters were allowing some real objects to get CR
   flags by astrodrizzle when only two images were available.
   Now, whenever there are just 2 WFC3-IR frames, we remove any CR
   flags, except in cases where *both* exposures have a 4096 flag.
   This allows us to catch the hot pixels that are not otherwise
   flagged, but not any true CRs.
  * improved the formatting of the help text.