Added code highlighter for built-in manual. Ported from Hackety Hack.
Replace Hpricot with Nokogiri, since Hpricot is officially dead. shoes/shoes4#176

Besides lib/shoes/help.rb I also updated samples/expert-funnies and the Japanese
manual since they both referred to using Hpricot, however they both depend on 'download'
which seems to be (currently) unimplemented.