Galaxy modelling code revision

Revision of the galaxy source modelling code, including the following changes:
+ Implementation of the spatial and velocity distribution models as `astropy.modeling.Fittable1DModel`
+ Use of `astropy.units.Quantity` throughout the code
+ Added tests
+ Added minimal docs
This test fails because the `data/atnf_sample.txt` file is not installed:
https://travis-ci.org/gammapy/gammapy/jobs/30936682#L974

You probably need to add this file via `get_package_data` in `setup_package.py` as described [here](http://astropy.readthedocs.org/en/latest/development/building.html#customizing-setup-build-for-subpackages) ... you can find a few examples in other parts of Gammapy or Astropy.