Added a batchFile step for Windows-based jobs

Pretty much as in the title. I wrote a test for it, which passes, and I made sure I could create a test job from this DSL:

```groovy
job {
  name "batchFile-test"
  steps {
    batchFile("echo hello world")
  }
}
```

As an aside, the project structure you guys have for this with Gradle was an absolute joy to use. I cloned the repo, and had my new function working within an hour, despite having extremely minimal Groovy experience, and never having used Gradle before.