Ignore Jekyll `_site' directory
Clean up code

* Re-indent and re-arrange in preparation for splitting into sub-files
* Move logic out of HTML
* Add `preventDefault()' to click event to avoid a page reload; routing
  works again
* Remove implied <html>, <head> and <body> tags (I know, I know)
Correct readonly Boolean attribute on <input>

* See [1]; page now validates as text/html

[1] http://www.whatwg.org/specs/web-apps/current-work/#boolean-attribute
Move JavaScript out of page

* Squash unused code in /script.js
* Fix minor errors (missing semicolons and undeclared variables)
Move stylesheet out of page
Use a (slightly) more consistent JS coding style

* Not necessarily more logical or useful
* <http://contribute.jquery.org/style-guide/js/> is half right
Shorten element ids
Neaten up HTML element attributes

* Order more consistently (id first)
* Place multiple attributes on separate lines
* Change XML-style <tag />s to HTML void <tag>s
* Remove implied type="text" attributes on form <input>s
Move input and output to a <nav> and an alert box
Refactor code to remove top-level functions
Factor out element references
Make <nav> slightly friendlier to small displays