Write the OpenOffice content file in one call.
Lazily load the OpenOffice doc file. As with Excelx, this means the OS or user will be responsible for cleaning up the temporary files.

Inline OpenOffice#extract_content.
Drop Base#load_xml as it is no longer used.
Inline and simplify OpenOffice.process_zipfile
Simplify Excelx#process_zipfile.
Drop our Roo::ZipFile abstraction in favor of more recent dependencies.
Extract Roo::Font to replace equivalent uses in Excelx and OpenOffice.