Move documentation for `tolerance` parameter to correct function

The `tolerance` argument belongs to `all_world2pix`, not
`all_pix2world`. It had been misplaced, in a typographical or merge
error.
Merge pull request #1895 from lpsinger/all_world2pix_doc

Move documentation for `tolerance` parameter to correct function
Display a better error message if the _wcs C extension can not load.
Add CHANGES entry.
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
Clarify CHANGES.rst
Merge pull request #2077 from mhvk/time-ut1-leapsec

UT1->UTC conversion wrong on leap seconds
Merge pull request #2088 from mdboom/testing/commandline-help

Clean up `setup.py test --help`