handle element definition properly; we need to do something with 
xmlElementContentPtr...

handle character encoding for inputs that aren't 8 bits wide (only SAX 
still needs to be done).

we need something similar to HTML.Parser...

we need to support html mode parsing in SAX

Node resulting from XMLReader.expand() does not get destroyed after 
next read(). This could cause real problems.

docloader function should be stylesheet local, not global to the whole module.
not sure what I was thinking...

add a variant of Stylesheet->apply() that allows passing of attributes. This 
will save us from having to mutex the set_attributes/apply process when using 
a shared stylesheet object across multiple concurrent requests.
