Updated the extension for Behat 3.0

This updates the extension to support Behat 3. Note that the ClassGenerator requires the upcoming 3.0-RC2. It won't work with the RC1 release (it requires https://github.com/Behat/Behat/commit/14bc1e4daabdd57d4d0eda0a090fece9a161e352)

BC breaks for users:
- the KernelAwareInterface has been renamed to KernelAwareContext
- the configuration for the bundle changed. It is not in the extension config anymore but in the setting of the suite

TODOs:
- [x] update the doc
- [x] update the tag for the mink driver once the MinkExtension is refactored fully for 3.0 (I will do it after sleeping, given that it is already 3am here)
- [x] write a real set of features and/or specs instead of relying on an example for Travis (which does not cover all features of the extension and will not fail cleanly when the extension gets broken as it does not describe the extension)