Links for section headings

Encouraged by some helpful user feedback, I've modified the HTML version of the Ruby on Rails Tutorial book to make links out of the section headings. This way, if you want to share a link to, say, Section 2.1, you can just right-click on the heading and select "Copy Link Location" to copy the URL into your clipboard buffer. (You could share section links before, but you had to navigate up to the Table of Contents and use the link there, which is inconvenient.) I hope this makes it easier to share and to link to any sections of the book you find particularly useful.