mvp-sample with security

Petter, 

I have a working example of form-based authentication in `mvp-sample` now.  

Later I will update the project to demonstrate how one can secure the navigation tree (filter content based on role).  But likely I won't get around to that until later.

The trip-up for people is likely the little known and documented static servlet prefix `vaadin.static.servlet.params`.  You must declare an init param for your custom widgetset in `application.properties` or `application.yml`; e.g.,

`vaadin.static.servlet.params.widgetset=org.vaaadin.spring.samples.mvp.ui.CustomWidgetSet`

I will add some notes in the `mvp-sample` README, so there's going to be one more commit on this branch.