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.
You would think that would be the case alas it seems not to be. Using that option for me caused it to produce some tags with no closing what so ever which broke the spec.