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.
this can now be reverted indeed. To support assertion counters in junit reports, I had a `resetStepCounters` and `resetAllCounters` method. Since it was to hard to support the counters, I've removed junit support for it, but forgot to revert this change.