Add ContextIntegrationTest, courtesy of @abayer
Do not set allowEmptyArchive if it is not specified

This element is incompatible with jenkins <= 1.480

See discussion here: https://github.com/jenkinsci/job-dsl-plugin/pull/131
Merge pull request #135 from tagged/archive-artifacts-context

Do not set allowEmptyArchive if it is not specified
Adding support for Maven pre/post-build steps