Rename

generate_html ->
generate_xhtml_files
Rename

make_tmp_dir ->
make_epub_directory
Reduce arguments of make_epub_directory
Add instance variables and use them to simplify code
Align
Reduce arguments of generate_xhtml_files
Reduce arguments of copy_static_files and clean up it
Reduce arguments of generate_contents_file
Rename

generate_contents_file ->
generate_contents_opf