Added a proper Behat suite for the extension

Instead of running the suite for the example app on Travis and expecting it to pass, which does not detect cases where the loading of features fails as with the Behat 3.0.0-RC2 tag, the extension has a proper suite which will run the testapp suite in different ways and assert the output.

Note that going from a successful build to a failed build is expected, because of the Behat bug https://github.com/Behat/Behat/pull/430 (this is actually how I detected the Behat bug, by doing the output assertion manually when looking at Travis)

Regarding the structure of the feature files, I'm not entirely satisfied with them, as they rely on the testapp and so they are not self-contained, but I don't know yet how to make them self-contained while keeping things readable. I will need to experiment a bit more with it.