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
Merge pull request #1892 from mwcraig/patch-5

Fix typo in doctest
Add more modeling Parameter unit tests
Add more modeling.core unit tests
Fix print_code_line to respect the UNICODE_OUTPUT configuration variable.
typo fixes pointed out by @mdboom and @pllim
Change language such that affiliated packages should go to astropy-dev
typo fix and minor clarification
Normalized wrapping to 72 chars
Merge pull request #1996 from eteq/update-overview-affiliated

Update overview discussion of affiliated packages
Clarified in docstrings that named tuples were being returned as suggested by @cdeil
Add a negative examples case to the docs as suggested by @cdeil
Simply absolute value-taking for dms as suggested by @mhvk
Changed `absdms` to `signed_dms` as suggested by @mhvk
don't run doctests with -t option
Update bundled six to 1.5.2
Fixed bug in Model2DKernel