[JENKINS-16824] log who's interrupting executors.

This bug indicates that people are having trouble understanding who's
responsible for aborting the build. This change records that
information through the logger.
[JENKINS-16845] Refinement to hotfix: if build field has already been cleared due to earlier warning, do not throw a new NPE.
[FIXED JENKINS-17402] getItem may return null but may not throw IAE.
fixed typo
Added the set-build-parameter command.
[maven-release-plugin] prepare release jenkins-1.513
[maven-release-plugin] prepare for next development iteration
updated changelog for release
updated changelog as a part of the release
merged back the RC branch
creating an RC branch
the trunk is toward 1.515-SNAPSHOT
Prevent ClassCastException when encountering ItemGroup of
non-TopLevelItem.

This is the case for instance when encountering a MavenModuleSet which
is ItemGroup<MavenModule>