Adding some starting points for testing the controller

How do you feel about including some starting points for unit and integration testing to the guide?

It took me a few hours and much googling of blogs to figure out how to do this in one of my own applications so I believe it would be useful to include some notes on how to test Spring Boot Web applications at the end of the guide. I've made my changes to the 'complete' project.

I can't seem to get gradle to work in my environment so `complete/build.gradle` may not be quite right.