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.
I had to revert this back, since junit needs to reset the counters (the counter features are added again). Junit reports the counters per scenario, while the current formatters only report the total count.