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.
Removing this interface would mean there is a listener in Output\Node\EL\AST that only works with JUnit. Should I move it to jUnit?