Added DslFactory

Allow typed access to JobParent through an interface for DSL script helper classes and to ease unit testing of those.
It's just a variable name here, it can have any name. I changed the name here because the object can also create view and folders, so it's no longer only a factory for jobs.

With the new release you can write typed helper classes, so in this case you could do the following if you want more IDE support:

```groovy
// helper class
import javaposse.jobdsl.dsl.DslFactory

class BuildFramework {
    static ant(DslFactory dslFactory, String jobName, String tasks) {
        dslFactory.job {
            name jobName
            steps {
                ant(tasks)
            }
        }
    }
}

// DSL script
import javaposse.jobdsl.dsl.DslFactory

BuildFramework.ant(this as DslFactory, 'project-a', 'clean test')
```