Formatter JUnit

The work on this Junit formatter at https://github.com/Behat/Behat/pull/527 seems to have stalled. I have take what was done by WouterJ done a refactor on the xml creation and completed the work. I have also brought the base inline with master so it can be merged.
We already have `ScenarioLike`, `ScenarioNode` and `Scenario` across the project. I don't want to introduce new UL term - `ScenarioElement`. Could we rename it to `ScenarioPrinter`?