Allow for `kind` kwarg to Table.argsort
Use mergesort alg for groups sorting to maintain order
Fall through to default quicksort if mergesort fails
Add unicode and latex aware liter symbol
Don't install wcs header files that aren't part of the API.
Reorganize units docs to emphasize Quantity over Unit.
Elaborate on uses of CDS unit standard
Use a mixed-unit example for Quantity-instantiated-by-list
Fix undefined reference target errors
Fix rst doctests in time package
Fix rst doctests in nddata package
Fix rst doctests in io package
Fix rst doctests in io.ascii
Remove extra output section
Update comments to match change in example.
Fixed sign of MexicanHat(1/2)DKernel and updated unit tests.
Fixed sign on MexicanHat2DKernel in docs/convolution/kernels.rst
Improve the coverage of the astropy.cosmology unit tests

* Use the coverall output to add some unit tests for interesting,
  untested functions and branches.
* Add a Tnu method like Tcmb.
* Add a few missing np.exp (replacing exp) for array arguments.
* Closes #1653
Corrected size of empty result array. Fixes #1603
Added Airy2DKernel to testing and changed import order.