Enhance sample application to demonstrate end to end scenarios

[Martin-Bohring]

Currently the sample application only demonstrates using fixieSpec with a simple sample POCO domain. This allows to demonstrates how to write specifications, but avoids all problems showing up in practice.

To show how to use specifications as integration tests, the sample application needs to to have persistence, an API, a UI and other port types.

• Add uses cases and specifications driven by REST API
• Create UI and specifications on top of REST API
• Document the sample more extensively, so a first time user can get around
• Simplify, simplify
• Eventually move the sample application to its own repository