[Property] Expose a new property to items that contain their DIDL descri...

...ption

- Add a new item property called Metadata which contains the DIDL-Lite XML
description of the item information. This information could then be passed
directly to renderers by dLeyna-renderer.
- doc updated.
- Fix issue https://github.com/01org/dleyna-server/issues/25

Signed-off-by: Christophe Guiraud <christophe.guiraud@intel.com>