Manage a stable branch

Merge "master" branch on a "stable" branch when:

• all permanent checks (downstream jobs of http://qa.nuxeo.org/jenkins/job/nuxeo-master-fullbuild-permanentCheck/) are fine
• a release from master is performed
• nightly tests (IT-*) are successful and the master branch SNAPSHOT artifacts are published.

When snapshots are published, set a tag on the stable branch (ie "5.6-SNAPSHOT").

Benefits:

• there's a tag on stable branch corresponding to the public SNAPSHOT artifacts.
• stable branch will host code (almost) synchronized with publicly available SNAPSHOT artifacts (or latest release); whereas the master branch contains code ahead of public snapshots
• when a functional test is broken, CI will be able to provide the new changesets (and corresponding diff) between current code and the latest stable code.