Adding nuskbygd

Added the full nuskybgd to the nustar-idl repo. Note that nuskybgd is NOT stand alone and requires bits of the AstroLib to run. You must check pull the entire nustar-idl repo for this to work.

Code originally written by Dan Wik. Current reference is here:
 http://adsabs.harvard.edu/cgi-bin/bib_query?arXiv:1403.2722

Added to git by BG on 2014/08/25