Added <fn> to DTDs

[Mark Giffin <mark@markgiffin.com>]

I am in the process of updating the LWDITA DTDs. I just added <fn> to 
topic.mod. I added fn to the all-inline content model used by shortdesc, 
p, pre, ph, dt.

If you want to see a diff of the changes, it is here:

https://github.com/oasis-open/dita-lightweight/commit/91dfc9143d2ef5f4ba744f64de6c035d47f836cf

I will also soon add <note> per recent discussions, and the 
specialization elements and attributes (spec*).

I had a couple of questions:

1. In DITA 1.3 <fn> includes the %select-atts; attributes. I left them 
out, do we need these in LWDITA?
2. I finally removed <object> from the DTD. I notice that <audio> and 
<video> still have object in their class attributes like this: "+ 
topic/object h5m-d/audio " -- will this cause any problems given that 
object is not defined?

Thanks,
Mark Giffin
Mark Giffin Consulting, Inc.
http://markgiffin.com/