Directories in packageMapping #6

As describe in #6 (Directories in packageMapping) I implemented the API, which can be used:

```scala
linuxPackageMappings += packageMapping(
   (target.value / "universal") -> "/opt/universal"
) withContents()
```
--

```scala
linuxPackageMappings <+= target map { target => 
  val src = target / "universal"
  val dest = "/opt/universal"
  packageDirectoryAndContentsMapping(src -> dest)
}
```