Wip/autoplugins

This is a work-in-progress pull-request. Feel free to add suggestions, corrections and ideas for the autoplugins.

## AutoPlugin Dependency Hierarchy

```
SbtNativePackager
    UniversalPlugin
        WindowsPlugin
        DockerPlugin
        LinuxPlugin
            RpmPlugin
            DebianPlugin
                DebianNativePackaging
                JDebPackaging
    JavaAppPackaging
        AkkaAppPackaging
        JavaServerAppPackaging
```

## Keys

For backwards compatibility there are all Keys inside `packager.Keys` or in the reference `NativePackagerKeys`.

All keys are defined in traits. The `autoImport` objects of the respecting plugin extends these traits
and define addition configurations if necessary. The structure should always look like this

```scala
object DockerPlugin extends AutoPlugin {

  object autoImport extends DockerKeys {
    val Docker = config("docker") extend Universal
  }

}
```

## Archetypes

I included the archetype declarations. Mostly because I'm not sure how to enable plugins in a scala (multi) build file.
Yeah, this feels right.