Support for Build Pipeline Plugin

This adds support for the build pipeline view and the build pipeline trigger.

```groovy
view(type: BuildPipelineView) {
    // common view options not mentioned
    displayedBuilds(int displayedBuilds)
    title(String title)
    selectedJob(String selectedJob)
    triggerOnlyLatestJob(boolean triggerOnlyLatestJob = true)
    alwaysAllowManualTrigger(boolean alwaysAllowManualTrigger = true)
    showPipelineParameters(boolean showPipelineParameters = true)
    showPipelineParametersInHeaders(boolean showPipelineParametersInHeaders = true)
    refreshFrequency(int seconds)
    showPipelineDefinitionHeader(boolean showPipelineDefinitionHeader = true)
}

job {
    ...
    publishers {
        ...
        buildPipelineTrigger(String downstreamProjectNames)
    }
    ...
}
```

I renamed DslScriptSpec to JobParentSpec, which should have been done in #107.