Implementation of wcs_to_celestial_frame
Minor improvements
Make wcs_to_celestial_frame extensible
Added tests and docstring for ``wcs_to_celestial_frame``
Skip doctest
Added changelog entry
Use context manager to register custom frame identifiers
Use a registry of known formatter classes.  This has the potential advantage of enabling user-defined formatters.  astropy.units.format.__init__ should still import all the built-in formatters to ensure they are added to the registry, so this is a minimal change.
Allow unit format names to be case-insensitive (normalizing to lowercase) and include a type check in case the format is not a valid string.
Ensure no Python modules under astropy/utils/compat are excluded from the source distribution
Added a changelog mention of this, though it won't be of interest to most users (packagers maybe though)
Following on from #2849, it became apparent to me that astropy.utils.misc was getting a bit cluttered, so this introduces a few new astropy.utils modules so that astropy.utils.misc could be broken up a bit.
Add a test from pyfits that was never copied over for some reason (https://github.com/spacetelescope/PyFITS/blob/master/lib/pyfits/tests/test_core.py#L718)
Remove Csv comment character to fix regression from #2812
Merge pull request #2860 from taldcroft/csv-regression-2812

Change Csv comment character to None to fix regression from #2812
Updates to the old PyFITS FAQ to better serve as an 'astropy.io.fits' FAQ, as more people are starting out using Astropy.  Removed a few questions mostly related to installation that are already covered generally by the Astropy installation docs.
Also make some slight updates to the docs on warnings
Merge pull request #2858 from embray/fits/missing-test

Add missing test from PyFITS
Added a changelog entry
Merge pull request #2857 from embray/utils/reorg

Reorganize astropy.utils.misc a bit