Added <fnref> to DTD

[Mark Giffin <mark@markgiffin.com>]

I added <fnref> to the DTD per the lwdita email list discussion here:

http://markmail.org/message/67sn3oxx4ht644de

Here are the changes:

https://github.com/oasis-open/dita-lightweight/commit/f0f710e9a540460018c660bfcc27a89d29d3834b

QUESTIONS FOR MEETING:
I had questions about the <fnref> attributes. <fnref> is specialized 
from <xref> but I did not add the @format and @scope attributes. Do we 
need these for <fnref>? If so, what values should they allow?

Thanks,
Mark Giffin
Mark Giffin Consulting, Inc.