This provides a more reliable way to determine that we are definitely imported from an astropy source checkout, by checking for a file .astropy-root that would only ever possibly exist in this case.  This is inessential though and can be removed if people want to get rid of this extra top-level file.
Actually this is better; we can put the special .astropy-root file under static/ so that it's not cluttering up the actual root.  But this is good enough for the desired purposes.
Add an explanation to .astropy-root
If extension modules need to be recompiled, automatically try running build_ext --inplace rather than just failing to import--very convenient.  At some point this should be extended to work for any extension module (rather than just using the utils._compiler heuristic) but that could be a little trickier
Add docstring for WCS.has_distortion
Fixed support on Python 3, and got rid of .astropy-root per https://github.com/astropy/astropy-helpers/issues/107#issuecomment-64282919
Fix problem where URL is being downloaded for each guess
Add Ellipse2D model
Add Ellipse2D model formula
Add test for Ellipse2D model
Add Ellipse2D example
Removed Ellipse2D __init__
Add changelog entry [skip ci]
Add funcsigs module to utils/compat
Minor typos in `docs/modeling/new.rst`

A few very minor typos I encountered while reading the docs.
Merge pull request #3158 from barentsen/patch-2

Minor typos in `docs/modeling/new.rst`
Merge pull request #3151 from Cadair/funcsigs

Add funcsigs to extern for Python 2 compatibility with the inspect module
Move the body of extern/six.py into a function so that any variables it uses (namely mod_info, which includes an open filehandle) go out of scope when they're no longer needed.  Fixes #3156.
Merge pull request #2001 from taldcroft/io-ascii-2001

Reading table with URL is doing many requests
Add machinery for SkyCoord to auto-search tables for appropriate columns