Allow for `kind` kwarg to Table.argsort
Use mergesort alg for groups sorting to maintain order
Fall through to default quicksort if mergesort fails
Merge pull request #1645 from mdboom/doctest/constants

doctest/constants
Merge pull request #1643 from mdboom/doctest/coordinates

fixes in coordinates to make doctests work
Merge pull request #1646 from mdboom/doctest/config

doctest fixes in config (incl. configs->config)
Merge pull request #1648 from mdboom/misc/dont-hardcode-sys-stdout

Don't hardcode sys.stdout at import time
Fix Sphinx warnings
Merge pull request #1655 from astrofrog/fix-warnings

Fix Sphinx warnings on Travis
Force stable sort for grouping
Merge pull request #1599 from taldcroft/table/group-mergesort

Force stable sorting of table in group_by()
Update CHANGES.rst

Remove unnecessary changelog entry.  See https://github.com/astropy/astropy/pull/1613/files#r7163815
Include "megabit" and "megabyte" etc., as units.  Fix #1543