Gradle integration for Jenkins

[Cervator]

What does this PR do?

Adds Gradle integration (via wrapper) for use with Jenkins code (javadoc, code completion, etc). Affects both the init scripts directory in the plugins image as well as a new src dir for writing assorted other scripts.

Also includes a Groovy Wrapper to execute Groovy without the Gradle hookup or a local Groovy install. To use simply run something like ./groovyw some.groovy

There is an attempt to avoid line ending issues in Vagrant when running on a Windows host OS as well.

Why did you take this approach?

Makes writing Groovy scripts for Jenkins and its plugins waaaaaay easier. I've used this approach myself years ago and am pulling in some stuff from https://github.com/tdongsi/jenkins-config as well, with more coming.

PR is more for an overview than merging for now. Includes prior work as well.

Contribution checklist

• THERE ARE NO UNENCRYPTED SECRETS HERE
• The branch is named something meaningful
• The branch is rebased off of current master
• There is a single commit (or very few smaller ones) with a Good commit message that includes the issue if there was one
• You have tested this locally yourself