Add support for conditional-buildstep-plugin

With transparent support for single and multiple steps, restricted
options for run conditions and build step runners, and nesting of
arbitrary build steps, rather than hardcoded to just support a few.
Add testing for each condition and runner.
Adding error condition tests for conditional steps
Merge pull request #128 from abayer/conditional

Add support for the conditional-buildstep-plugin
Add conditional buildstep support for status
Merge pull request #134 from abayer/add-status-condition

Add conditional buildstep support for status
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
Merge pull request #136 from abayer/maven-pre-post-build-steps

Adding support for Maven pre/post-build steps
allow to specify the Maven installation