[nested-sets] support nested handler sets (not deep)
Merge pull request #750 from portertech/nested-sets

Support nested handler sets (not deep)
[0.13.0] use sensu-* 1.0.0, sensu-em 2.4.0, updated license date for the release
[0.13.0] updated release date in changelog
release sensu 0.13.0, to be packaged and documented
Merge pull request #751 from portertech/0.13.0

Final 0.13.0 release prep