Fix typo in doctest
Raise a NotImplementedError when fitting composite models.
clarify error message
add changelog entry
use correct PR number
Add test for array coordinate length (issue #1761)
Add len() to coordinates

Closes #1761
Add tests of len() for scalar coordinate and length 1 coordinate
Add changelog entry for 1761
Correct the change log message
Move change log entry to correct section
Started replacing several interfaces with cross-compatible versions from six.  Still most tests failing.
Gets the remaining of the io.fits tests passing nativily in Python 2 and 3.
Remove any messing with builtins
Fix an issue I was specifically having on Python 2.6.  I'm not sure why this hadn't come up before, except that maybe it's been fixed in newer 2.6.x versions...?
Fix a few stray Python 2-isms that crept in to new code since I first started this branch
Merge pull request #1994 from mdboom/testing/better-warning-about-no-rst-on-py3

Present a better warning about no .rst doctesting on Python 3
Merge pull request #1301 from embray/fits-python-3

Remove io.fits dependence on 2to3
Merge pull request #1917 from mwcraig/fix-1761

Add len() to coordinates
Merge pull request #1915 from nden/error-fit-composite

Error fit composite