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.
Merge pull request #49 from stof/proper_feature_suite

Added a proper Behat suite for the extension