The last jUnit formatter PR?

This replaces #618  I think we're very close to completion (thanks to the amazing work by @james75 )

Remaining questions
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* https://github.com/Behat/Behat/pull/618#discussion_r22332553 (not sure if anything is needed there)
* https://github.com/Behat/Behat/pull/618#discussion_r22332177