A few tweaks for ReSpec

Hi everyone (and especially Robin),

I made a few changes locally to ReSpec that I think are improvements,  
and I'd like the ok to check them in.

Here's what I did:

* Implemented <termdfn> and <term> elements, to generate properly  
styled definitions of terms in prose, and to allow uses of a defined  
term to link to its point of definition.

* Made xml:id get copied only as id and not as both xml:id and id to  
passed-through xhtml elements. This is more consistent with generated  
xhtml elements, less likely to cause validation issues, and the only  
right thing in case of html output.

* Changed copyright and registered trademark symbols to use numeric  
entities instead of being included as raw unicode to avoid running  
into charset issues.

Here's the patch:

OK if I land this? I know less than zero about xslt so I don't know  
if I did things the right way.

Regards,
Maciej