Adds initial predicted counts cube script
Adds unit test outline (incomplete)
Adds demonstration script to gammapy-examples
update
Fixes compatability issue with gammapy-datasets
update
Add energy_bin_centers_log_spacing utility function
Update npred computation and example with Ellis's latest version
Fix effective area test
Implement spectral cube coordinate and solid angle image methods
Merge pull request #149 from cdeil/fix_effective_area_test

Fix effective area test
Continue implementation of spectral cube npred computation
Merge pull request #150 from cdeil/npred_cube_fixes

Npred cube fixes
Implemented distributions as astropy models

Conflicts:
	gammapy/astro/population/spatial.py
Changed all models to astropy models added testing and docs
Changed code to use quantities