Implement unit usage into astropy.cosmology

* Distances, etc. now returned as astropy.units.Quantity objects
* Some cosmological variables (Tcmb0, H0, etc.) stored as quantities
* Unit tests updated to reflect use of quantities.
* coordinates.Distance makes use of cosmology object, so those uses
  updated.
Update documentation for units in the cosmology sub-package.
Speedup for angular_diameter_distance_z1z2, due to @nhmc
Update funcs docs to give Quantity return type
Change distmod to use units.mag, remove c_kms

* Following suggestion by @nhmc, use constants.c directly instead
  of c_kms
* For consistency, distmod also returns a quantity object.
Comments from Michael Droettboom.

* Mostly minor documentation and line break changes.
* use Quantity(blah, u.Mpc) instead of Quantity(blah, 'Mpc')
Merge pull request #1237 from aconley/cosmology_units

Cosmology units