Rewrite for Behat 3

**This PR isn't ready to be merged**

As the architecture of Behat 3 is different of Behat 2.5, we need to do some changes inside this extension.

We should discuss how this PR should me merge in your repository as it break the compatibility with Behat 2.5

- [x] Remove xml declarations (to match the MinkExtension architecture
- [ ] Rewrite documentation
- [x] Fix phpspec test

We can't keep the NamespacePass as we do not have the behat.context.class anymore