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')
Handle any exception the unit parser might throw our way.
Fix bug parsing exponents without mantissas in CDS parser.
Add test for CDS parser
Merge pull request #1237 from aconley/cosmology_units

Cosmology units
Set some tests to run with an ascii encoding.
Merge pull request #1288 from mdboom/unit/handle_exceptions_in_parser

Another unreadable VO table: unit parsing failure
Merge pull request #1098 from mdboom/misc/travis-test-with-ascii

Set locale to C on Travis