Replace Hpricot with Nokogiri, since Hpricot is officially dead. shoes/shoes4#176

It seems the code for generating the manual (lib/shoes/help.rb) is entirely untested.
My initial attempt to start adding tests for it failed, so I implemented this as
a sprint to be redone and verified with tests. However comparing the generated
HTML in a browser seemed to look good :D

There's a bug in the 'Pure Java' Nokogiri (>= 1.5) when adding raw html to the
document. I went back to the FFI version (~> 1.4.0) and that works, but JRuby-lint
depends on 1.5 so I had to comment that development dependency out for now. Will
try a minimal example of the bug and see if this is already reported upstream.

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.