#37: Added the MuisModel placeholder interface.
Added style sheets into the MuisHeadSection class, since that's where
they're specified.
Moved MuisDocument out of the MuisDocumentStructure--this is more of a
pure structure now.
Refactored template XMLs to be just like main MUIS documents.  Warnings
are given if title or style sheets are specified (style sheets might be
used later) and errors are thrown if there is anything but a single
content widget in the body element.
#37: Added a little bit of parsing code for models