Added a valid JUnit Formatter

As current JUnit Formatter output does not validate against this JUnit xsd: https://svn.jenkins-ci.org/trunk/hudson/dtkit/dtkit-format/dtkit-junit-model/src/main/resources/com/thalesgroup/dtkit/junit/model/xsd/junit-4.xsd (This one is used by Behat), 
I'm rewriting it :wink: along with its behat scenario and specs.

See https://gist.github.com/gquemener/8582134 for more information about validation problems.