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
- it is not possible to use ``@AcmeDemoBundle/Features/greet.feature`` as locator anymore. Instead, you should use ``Features/greet.feature`` as locator, and use the ``-s`` option for the suite corresponding to the bundle.

TODOs:
- [ ] update the doc