Turned downloads.py module into a package with several modules.
Created FocalPointCalculator class to encapsulate the process of 
determining the points of interest for a source.
Added a TripletFocalPointCalculator for calculating the focal points of a 
triplet layout of images.
Added save method to DownloadedFitsImage.
Merge branch 'master' into download_triplets
Merge pull request #170 from drusk/download_triplets

Download triplets