Optimize: Allow rules to break error-loop (x2 faster)

An improvements ON TOP of #182 optimizations currently underway.
Added a new _BreakLoopException that is thrown by the ``$ref``` rule, and removed the ad-hoc logic at the beggining of the error-loop.

* Not only faster, but better standard-deviation --> probably more memory-efficient.
* Expecting the usual TravisCI problem with python-2.7.6 (pypy)