RENAMING MODULES :
sndrizzle.py ==> pipeline.py
sort.py ==> exposures.py

  - renamed primary pipeline function to singlepipe
  - renamed sort.into_epochs as exposures.copy_to_epochdirs
pipeline.py :
  - now saving the union badpixel mask that is constructed for each diff image
  - now constructing the composite IVM wht map for diff images.
tweaking the names again:
  singlepipe --> runpipe

Added verbose option for printing epochs list in exposures.
Merge remote-tracking branch 'origin/master'