SLING-3685 - Document and further automate the IDE tooling release
process

- enable the apache-rat-plugin as part of the default build, in the
verify phase
- add/correct license headers for all non-compliant files

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1605327 13f79535-47bb-0310-9956-ffa450edef68