Run `tidy` against `assets/changes.html`

Run HTML tidy[1] against `assets/changes.html` to improve the HTML
syntax:

- add a doctype
- add a document title
- terminate `<li>` lines with a closing tag

[1]:
https://developer.apple.com/library/mac/documentation/Darwin/Reference/Manpages/man1/tidy.1.html
Merge pull request #68 from mattbostock/tidy-changes-html

Run `tidy` against `assets/changes.html`