Rename `spectral_cube` to `data`
Rename GammaSpectralCube to SpectralCube
Move gammapy/data/core.py -> gammapy/data/spectral_cube.py
Make build for gammapy.data work
Add gammapy/data/event_list.py
Merge pull request #176 from cdeil/spectral_cube_to_data

Move `spectral_cube` to `data` sub-package