replaced constants in PseudoHiki::Format with class variables. they don't need to be accessed from outside
changed the style of defining class methods so that the documentation with RDoc could be properly generated
cosmetic changes: fixed the indentation of class method definitions
revised the explanation of options for PseudoHiki::Format.format()
revised the explanation of class methods of PseudoHiki::Format: ::to_html(), ::to_xhtml() and ::to_html5()