Implemented ZT-Normalization. More tests should be implemented.

bob/bio/base/pipelines/vanilla_biometrics/__init__.py

...bio/base/pipelines/vanilla_biometrics/abstract_classes.py

bob/bio/base/pipelines/vanilla_biometrics/pipelines.py

bob/bio/base/pipelines/vanilla_biometrics/wrappers.py

bob/bio/base/test/test_transformers.py

bob/bio/base/test/test_vanilla_biometrics.py

bob/bio/base/test/test_vanilla_biometrics_score_norm.py