Address problems with constraints reported in #1060 and #1064
Started documenting some details on how I maintain the bug fix branch(es).  Also added some details on how cherry-picking merge commits works, adapted from an e-mail I wrote some months ago.  Still need to describe the steps for preparing a bug fix branch for a release.
Mostly finished the dicussion of maintaining bug fix releases, and modified the release procedure to account for them.  There is a mention of 'updating the changelog' that is left hanging, but I will try to address that in #695.
Use repr() to format strings instead of putting literal quotes in the format string.
Adds directions for updating the changelog (and some justification for why it exists and why you should add to it)
Merge pull request #1104 from iguananaut/sphinx-windows-issues

Fix minor issue with build_sphinx on Windows
Fixed bug in WCS error message
Merge pull request #1108 from astrofrog/wcs/fix-error-message

Fixed bug in WCS error message
Merge pull request #940 from iguananaut/issue-940

Document releasing bug fix releases
Merge pull request #695 from iguananaut/issue-695

Write/improve guidelines for contributing the the CHANGES.rst changelog
Merge pull request #1107 from nden/constraints_issues

Address problems with constraints reported in #1060 and #1064

There's still a minor issue with constraints which is that after fitting the constraint attributes on the parameter objects are reset to the default values. But the model.constraints object has the correct values. I am going to merge this so it can be used and will open a different issue for this last problem I am aware of. It doesn't affect fitting but can be misleading.
update .travis.pip.wheel to support pure-python wheels
update .travis.yml to use wheels for sphinx and matplotlib