create_html_chapters breaks references between chapters

[shiffman]

create_html_chapters still thinks there is one long HTML document and so references like:

In section I.2 of the introduction blah blah

end up pointing to the current chapter, i.e.

http://natureofcode.com/book/chapter-1-vectors/#intro_section2

when they should go to:

http://natureofcode.com/book/introduction/#intro_section2