add support for the pipeline-stage-view API

[nergdron]

It'd be really nice to have bindings in this lib for the common pipeline-stage-view plugin, to be able to fully dig into multi-stage pipeline jobs:

https://github.com/jenkinsci/pipeline-stage-view-plugin/tree/master/rest-api

[thomaspeitz]

Hey @nergdron - Just starting using this project but pipeline steps of a job can bet retrieved through: pipeline.go functions/struct. If you use pipeline.go func (job *Job) GetPipelineRun(id string) function, you get through a jenkins build-nr all pipeline steps.