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
There is no policy for documentation of private functions.

Writing code / docs such that you think it would be easiest to understand for someone else (or yourself in a year) is best. But there's also the [80:20 rule](http://en.wikipedia.org/wiki/Pareto_principle) that says you should stop when it's in an OK state, which in this case, for well-structured code, means that it's OK to leave private functions without docstrings or just a one-line comment and instead move on to the next job.