Added code to the ContentHandler to extract the site informarion directly from the input file itself, rather than depending on it being passed in to the handle_page() method.
* added a mandatory titlesdb and optional language argument to the command-line parser
* factored a _get_rule_for_heading() method out of the parsed_structured_section() method that calls on a ditionary with rules and relations for given heading types
* added a sense_num rule to handle the use of sense numbers in definitions and cross-references.