TableRowParser::TD must be defined before TableCellNode#intialize()
removed a temporary variable in InlineParser#convert_last_node_into_leaf()
removed an unnecessary comment
removed unused accessors of BlockNode (#base_level and #relative_level_from_base)
refactoring of InlineParser#split_into_tokens()
removed a temporary variable in InlineParser.parse()
removed a temporary variable in BlockLeaf.create()