Add support for Jenkins Pipeline jobs

[cprice404]

With these commits, the plugin can now be used from Jenkins Pipeline jobs, by adding a line like this to your pipeline build script:

gatlingArchive()

[cprice404]

Fixes gatling/gatling#3018

[slandelle]

missing empty line?

[slandelle]

Awesome Chris, thanks a lot! We'll give it a try asap and release a new version.
Where did you find a migration documentation?

[cprice404]

Pushed up a commit with the proposed whitespace fixes.

@slandelle I didn't really find any comprehensive docs on migration; it was kind of a combination of asking questions in IRC and on the mailing list, looking at the source code of other plugins and of Jenkins itself, and just trying things out until I understood it. The closest thing to docs that I found was this:

https://github.com/jenkinsci/pipeline-plugin/blob/master/DEVGUIDE.md

That actually has most of the information, it's just written in a way that is pretty hard to understand until you get into the code. This also has some useful information:

https://github.com/jenkinsci/workflow-step-api-plugin

I was planning on writing some kind of blog post / tutorial about the upgrade experience to share with Jenkins and hopefully make things easier for the next person :) That's why I broke it into so many commits.

[slandelle]

[slandelle]

@cprice404 Could you please squash your commits into a single one?

[cprice404]

squashed.

[slandelle]

Awesome work, thanks!

[jglick]

Does not make sense to both make GatlingPublisher implement SimpleBuildStep (which makes it show up as an option to the step metastep), and create a dedicated Step. You can do the first if you have no special needs beyond “make what worked in a freestyle project also be available in a Pipeline build”; the second if you wanted to customize the options and behaviors available from Pipeline.

[cprice404]

@jglick is it harmful to support both? e.g. you can trigger the 'archive' functionality either way (though I recgonize that the code paths are slightly different).