[API] Add CreateReference method to org.gnome.MediaContainer2

Fix https://github.com/01org/dleyna-server/issues/7

Signed-off-by: Ludovic Ferrandis <ludovic.ferrandis@intel.com>
[Property] Add support for object.container.playlistContainer

- Add object class object.container.playlistContainer
- Add MediaServer2Spec definition "playlist" for it

- Rename MediaServer2Spec "playlist" to "item.playlist" for
objec class object.item.playlistItem

Signed-off-by: Ludovic Ferrandis <ludovic.ferrandis@intel.com>
[Property] Add support for object.container.storageFolder

    - Add object class object.container.storageFolder
    - Add MediaServer2Spec definition "storage" for it

Signed-off-by: Ludovic Ferrandis <ludovic.ferrandis@intel.com>