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.
These method names assume you'll always print tags using this printer. This limits that interface reusability, essentially meaning that you'll use this interface only once, creating a hollow abstraction. We need to either generalise this interface or drop it altogether. There's absolutely no need to have an interface for every kind of printer if we don't need them for reusability yet.