Implement ustring::substr. Patch by: konrad Review by: lg
Implement ustring::substr.
Patch by: konrad
Review by: lg


git-svn-id: c3edda122e248ef33995da003806c930f62b9468@1470 31904243-a364-42ac-a3db-9305396518ba

jeremyselan/oiio,bdeluca/oiio,mcanthony/oiio,scott-wilson/oiio,OpenImageIO/oiio,lgritz/oiio,scott-wilson/oiio,micler/oiio,scott-wilson/oiio,micler/oiio,YangYangTL/oiio,cwilling/oiio,mcanthony/oiio,OpenImageIO/oiio,YangYangTL/oiio,bdeluca/oiio,cwilling/oiio,OpenImageIO/oiio,micler/oiio,micler/oiio,mcanthony/oiio,sambler/oiio,bdeluca/oiio,lgritz/oiio,sambler/oiio,lgritz/oiio,sambler/oiio,jeremyselan/oiio,sambler/oiio,mcanthony/oiio,lgritz/oiio,OpenImageIO/oiio,YangYangTL/oiio,cwilling/oiio,jeremyselan/oiio,cwilling/oiio,bdeluca/oiio,YangYangTL/oiio,scott-wilson/oiio