remove head.min.js in favor of simple built-in script loader

demo.html

index.html

js/reveal.js

lib/js/head.min.js

plugin/markdown/example.html

test/assets/external-script-a.js

test/assets/external-script-b.js

test/assets/external-script-c.js

test/assets/external-script-d.js

test/examples/embedded-media.html

test/examples/math.html

test/examples/slide-backgrounds.html

test/examples/slide-transitions.html

test/test-async-dependencies.html

test/test-dependencies.html

test/test-markdown-element-attributes.html

test/test-markdown-external.html

test/test-markdown-options.html

test/test-markdown-slide-attributes.html

test/test-markdown.html