Accept application/json in GitHub change hook, accept pings

Conflicts:
	master/buildbot/test/fake/web.py
Add changes to GitHub change_hook to relnotes

Conflicts:
	master/docs/relnotes/index.rst
Merge pull request #1461 from sa2ajj/port-gh-1460

Port GH-1460: improve GitHub hook support
Correct expectation from interrupted master shell command

Refs #2985.
Document the `waited_for` argument to addBuildsetForXxx

Fixes #3097.
Merge pull request #1462 from djmitche/bug2985

Correct expectation from interrupted master shell command
Merge pull request #1463 from djmitche/bug3097

Document the `waited_for` argument to addBuildsetForXxx