Add gammafit to intersphinx; switch to astropy dev for intersphinx
Move URLs in docs to references.txt
Add crab and diffuse to top-level gammapy.spectrum namespace
Rename some things in gammapy.spectrum.crab
Rename Pion class to PionDecaySpectrum

Add test_pion.py and some docstrings.
Rename InverseCompton to inverseComptonSpectrum
Add test_inverse_compton.py file
Merge pull request #90 from cdeil/issue_90

Remove sub-module hierarchy for gammapy.spectrum