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.
IMO, the folder should not need to exist first. It is not consistent with the way other tools are working. for instance, PHPUnit creates the output folder if it does not exist