[TASK] Update PHP namespace

PHP namespace must not be TYPO3\CMS\Metadata as it
is a reserved namespace.
I suggest to be Fab\Metadata because it is simply
fabulous ;)

Change-Id: Ia76b0cb9a8eee62ea4601bb9df3dafe3fe7ca168
Releases: 2.0
Resolves: #60896
Reviewed-on: http://review.typo3.org/32094
Reviewed-by: Fabien Udriot <fabien.udriot@ecodev.ch>
Tested-by: Fabien Udriot <fabien.udriot@ecodev.ch>