update
Adds fermi_exposure.fits.gz exposure cube binary file
Changes load scripts to include new fermi exposure cube
Upates fermi dataset README file and adds executable script to outline commands used to generate the exposure cube
Fixes test failure
Merge branch 'exposure_cube2' of github.com:ellisowen/gammapy into npred_cube

Conflicts:
	gammapy/datasets/data/fermi/README.rst
	gammapy/datasets/data/fermi/fermi_exposure.fits.gz
	gammapy/datasets/tests/test_load.py
update