Cosmetics.

Lose a few commas.
Rename `grunt build` to `grunt dist`, to not confuse this with `grunt build:index`.
Extract `test` task from `dist` and `server`.
Add description to tasks.
Enable basic TravisCI testing for presentations.
Simplify API: Make html-slides work like md-slides. Hence both can use --attributes option.
Add `useSass` prompt for creating custom themes with SASS.
Support simple slides without a file, e.g. only an background-image.
Get rid of coffee-script as production dependency.
Merge branch 'dev'

* Add custom theme support using Sass.
* Simplify API: HTML-slides behave more like markdown-slides, i.e. both can use the `--attributes` option.
* A slide `Object` in `slides/list.json` does not need to have a filename to enable simple image slides.
* Rename `build` task to `dist` task, so it won't be mistaken for `build:index`.
* Remove `coffee-script` as a production dependency. Instead compile `.coffee` by running `npm prepublish`.