change default from np.median to np.ma.median
Cleaned up the changelog in prep for release.
Preparing release 0.4rc1
Back to development: 0.4rc2
Add 0.4 what's new section about coordinates

Also adds a  doc ref that was needed in docs/coordinates/representations.rst
updated coordinates to stable status
Update status for astropy.cosmology and astropy.convolution
Typo fixes from @mdboom's suggestions
added missing space

[ci skip]
Update coordinates What's New to emphasize positive new features

- Moves most of the original content into a new section "Migrating
  from pre-v0.4 coordinates" in the main coordinates index page.
- Adds a sphinx note up top pointing to the possible need for
  migrating code with a link to the new section.
Fix docs typo
Merge pull request #2680 from taldcroft/whats-new-04-coordinates2

What's New 0.4 coordinates - take 2
Merge pull request #2678 from astrofrog/update-stability

Update status for astropy.cosmology and astropy.convolution
Merge pull request #2677 from eteq/coordinates-stability

updated coordinates to stable status
Updated docstring to say that fields are not stripped
Update this test to provide a little better information when it fails.
Merge pull request #2682 from amras1/fix-tab-docstring

Changed docstring of tab-separated values reader
Added a note to system packagers, where hopefully they can find it
Fix how terminal size is handled in table.pprint.
Raise exception when running tests inside IPython.