Support for Nested View Plugin

New DSL syntax example:

```groovy
view(type: NestedView) {
    name('project-a')
    views {
        view {
            name('overview')
            jobs {
                regex('project-A-.*')
            }
            columns {
                status()
                weather()
                name()
                lastSuccess()
                lastFailure()
            }
        }
        view(type: BuildPipelineView) {
            name('pipeline')
            selectedJob('project-a-compile')
        }
    }
}
```

Replaces #175