Normalize dev.html and tests, remove lib/

This commit makes dev.html contain the same vendor scripts as the test
runner, and removes the remnants of lib/.

- gettext.js -- needed for I18N but this needs a full review before 2.0
                anyway
- json2.js -- can be provided by https://www.npmjs.org/package/JSON2 if
              needed
- showdown.js -- can be provided by
                 https://www.npmjs.org/package/showdown if needed
- wgxpath.install.js -- is already provided by
                        https://www.npmjs.org/package/wgxpath