* NodeIter methods to move index backwards and refresh the internal
   nodelist.
 * select_scorable will now backup and re-score converted <div> tags.
 * nodeiter is now refreshed after paragraph-izing.
 * Added 'readability-styled' class to paragraph-ized text, for
   consistency.
 * Cleaner now leaves <style> tags in place.
 * Altering of node to <div> now makes a copy.
 * get_inner_text now uses node.text_content, and trims the final string,
   so innertext lengths match those in readability.js to produce the same
   scores.
 * Fixed a routine which prunes out empty paragraphs.
 * Moved convert_brs method close to prep_document where it's used.
 * Added new debug statements to match those found in readability.js.
 * Removed a few verbose debug statements.