Add Bootstrap

To be able to create basic functionality as fast as possible, we need an UI framework. I thought about adding [Zurb Foundation](http://foundation.zurb.com/) to learn something new, but decided to stick with [Twitter Bootstrap](http://getbootstrap.com/) to avoid a steep learning curve.

I think this is the correct dicision, because when starting a project, we have to be able to do fast and flexible prototyping. As soon as a "real", sophisticated layout needs to be applied, we can still switch to another framework or build everything from scratch.