[xliff] Namespace for XLIFF

Hi all,

The OASIS TC guidelines recommend to specify a namespace identifier for
specifications. They have already a reserved namespace where such
identifiers can be set. At some point we should probably come up with a
namespace identifier for XLIFF.

This namespace should probably also be in the specification. For example the
namespace for the XML Catalogs format
(http://oasis-open.org/committees/entity/spec-2001-08-06.html) is
"urn:oasis:names:tc:entity:xmlns:xml:catalog".

The complete documentation regarding the OASIS namespace can be found at
ftp://ftp.isi.edu/in-notes/rfc3121.txt.

cheers,
-yves