add a method argument to _IncrementalFileWriter
implement a etree.htmlfile that passes OUTPUT_METHOD_HTML to _IncrementalFileWriter
write_declaration now throws an exception for html files.
don't write namespaces in html mode
modify write_declaration to fail for every document type except xml
add testcases for etree.htmlfile
dried htmlfile
Merge pull request #142 from plq/master

Implement etree.htmlfile