Update index.html.

* add `meta charset`
* specify lang
* remove default type for css
* use https for the GitHub and Twitter links
Update uglify to v2.4.13.
README.md: add link to the website and JSHint.
Make `textarea` resize only vertically.
master.css: Minor tweaks.
Add missing semicolon.
Switch to the minified underscore build.
Fix a few variable redefinitions.
Slightly improve markup.
Ignore _site.
Rename uglify.js to uglify.min.js.
Minify JSHint with uglify.

This should be automated eventually.
Add _config.yml to ignore some files from Jekyll build.
Enable strict mode.