Nitrous cloud service

Adapts the installation guide to include the setup using nitrous cloud service. Also adds some hints to the 'first app' guide for building the app using a cloud service.

Cloud services like nitrous allow users to build rails apps without having to install anything. This is useful for participants who do not have administrative privileges on their computer or have to use someone else's computer (e.g. if they do not have their own notebook). It also allows participants to continue coding their app from different computers, or collaborate on it without having to learn git. Also installation parties are optional if there is not so much effort to be spent for installation.

I've mainly put the information into the installation guide in order to keep the 'first app' guide short. What do you think about this approach?