Update testing docs in README
Update CONTRIBUTING.rst

"if anyone" -> "anyone"
Fix typo.
Add assignee attribute to pull requests.
Add @nvie to the AUTHORS.rst file
Allow removing milestone in Issue.edit()

Pass milestone=0 to remove a milestone.

This is new API from GitHub; they require passing a null milestone, so
change 0 to None after removing all the default None parameters from
the JSON dict.