add gal-sim: a galaxy simulation toolbox.

GalSim is a package for simulating astronomical images of galaxies.
It was developed to support the upcoming "Great3" challenge,
http://great3challenge.info/, to test different methods for measuring
weak gravitational lensing, a key way of measuring mass in the
universe through its bending of light.
put caveats in a separate caveats function
 umm
add conditional OpenMP support
single quotes -> double quotes
Merge branch 'add-galsim-2' into dev

Conflicts:
	gal-sim.rb