1.2.0: unreleased

This is a small release with some enhancments.

Features Added

• Organization Invitations Preview API is now supported. This includes an additional Invitation object. This is the result of hard work by Hal Wine.
• A ShortLabel class was added to represent the shorter (description-less) representation of labels returned by the API.
• The branch protections API is now completely represented in github3.py.