add build task. Close #17
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.