CD-1 issue #16: offset for attribute selector

The issue number refers to the XLS-sheet found in this email:
http://lists.oasis-open.org/archives/xacml/200909/msg00013.html

The commenter proposes that an attribute selector should include an 
additional xpath parameter which acts as an offset for the xpath of the 
selector.

I see the that this could be useful, but it is a fairly big change and I 
am uncertain about the implications so I propose that we push this 
feature past 3.0 so we can consider it carefully.

Best regards,
Erik