CustomKernel fixes
Added CustomKernel tests for zero-summed kernels.
Ignore differences between 32-bit and 64-bit ints
Fix equality for array coordinates

Closes #1832
Added test for #1832 failure mode
Add test for #2037 failure mode
update changelog
Get `Longitude` sign to come out right from `wrap_angle` when 0

Closes #2037
Fix test that incorrectly had '-0h00m00s 0d00m00s' and similar
Ignore long int suffix in doctests, e.g. 1234L
Display a better error message if the _wcs C extension can not load.
Add CHANGES entry.
renamed deriv method to fit_deriv
rename 'deriv' to 'fit_deriv' in documenation
Merge pull request #2061 from mdboom/wcs/missing-c-extension

Display a better error message if the _wcs C extension can not load.
Clean up `setup.py test --help`
Correct problems with UT1->UTC->UT1 roundtrip; add regression tests
added an entry in the changelog
Clarify CHANGES.rst
Make UTC the default time scale