Add Reference/API section label to docs
Add Crab HESS spectrum flux points as example data file
Move gammapy.astro to gammapy.astro.source

I think in this case splitting astro into sub-packages
(source, population) makes sense.
Clean up test_isrf.py a bit
Add gammapy.utils.distibutions which contains some code for random
sampling from arbitrary distributions.

This is old code that needs to be cleaned up.
Possibly it can be thrown out and we use ROOT or something
else for this instead.
Add gammapy.astro.population (old code from @adonath)
Add plot_spirals.py script
Import sub-module as * in gammapy.astro.population __init__.py
Update astro sub-package docs to reflect splitting in source/population