master.css: Minor tweaks.

* make `textarea` resize only vertically
* be more explicit with `background`
* use the `font` shorthand when possible
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.