modularize themes and convert them to sass (closes #191)

.gitignore

css/reveal.css

css/theme/README.md

css/theme/beige.css

css/theme/default.css

css/theme/serif.css

css/theme/simple.css

css/theme/sky.css

css/theme/source/beige.scss

css/theme/source/default.scss

css/theme/source/serif.scss

css/theme/source/simple.scss

css/theme/source/sky.scss

css/theme/template/mixins.scss

css/theme/template/settings.scss

css/theme/template/theme.scss

index.html

js/reveal.js