Make WCS instances use WCSAxes if available
Added simple documentation
Added changelog entry [ci skip]
Added URL for more details on WCSAxes
Make comment into docstring as suggested by @mhvk
Warn on invalid WCS unit
CHANGES entry
update setup_helpers and a corresponding setup.py change
skip doctest with matplotlib in skycoord for stability
Clarify what is returned
Merge pull request #3183 from astrofrog/wcsaxes-compatibility

WCSAxes compatibility hook
Merge pull request #3207 from hamogu/fix_3111

skip doctest with matplotlib in skycoord for stability
Remove workaround for Python 2.7.9 in .travis.yml
Merge pull request #3210 from astrofrog/remove-workaround

Remove workaround for Python 2.7.9 in .travis.yml
Merge pull request #3199 from mdboom/wcs/unit-warning

Warn on invalid WCS unit
Use union merge for changelog
update to latest astropy-helpers
Merge pull request #3211 from astrofrog/changelog-union

Use union merge for changelog
Merge pull request #3206 from eteq/update-setup-helpers

update setup_helpers and a corresponding setup.py change
Generate ERFA wrapper automatically