Rails engine

This PR converts the peoplefinder application into a rails engine.

Making it a rails engine will allow simple 'wrapper applications' which contain the engine (pulling it in as a gem). The wrapper applications will allow re-skinning of the application so that it can be used by different organisations.

This changes in this pull request fall under the following headings:

* removing configuration files and other elements which belong only in an application and not in a gem
* moving the models, controllers, helpers and other elements into a 'peoplefinder' namespace
* adjusting the tests to work with the namespace