Added hst-specific phot modules
 * added hstapcorr.py for measuring and retreiving aperture corrections
 * added hstapphot.py for HST-specific aperture photometry, accessible from
   the command line
 * modified aper.py to allow for simpler sky determination when the field
   is mostly empty (as in a diff image)
 * added various .dat files with standard star photometry, aperture corrections,
   zeropoints and the like.