ISSUE-89: ttm:role - allow multiple tokens [DFXP 1.0]

ISSUE-89: ttm:role - allow multiple tokens [DFXP 1.0]

http://www.w3.org/AudioVideo/TT/tracker/issues/89

Raised by: Glenn Adams
On product: DFXP 1.0

since ttm:agent can refer to multiple agents (as IDREFS), ttm:role should be able to specify multiple roles (as NMTOKENS);