Change default recipients list for builds

New rules:

1. If the build happens on the default branch, all members of the
repository are notified.
2. If the build is not on the default branch, the author and the
committer who are also members of the repository are notified.
Rework email event handler specs

Stubs are re-examined and fixed.
Add enough stubs to instruments spec
Remove unnecessary stub
Merge pull request #291 from BanzaiMan/core_gh94_email_notification_revamp

Change default recipients list for builds