Merge branch 'cleanup' into 'master'

bob/learn/tensorflow/callbacks.py

bob/learn/tensorflow/layers.py

bob/learn/tensorflow/losses/__init__.py

bob/learn/tensorflow/losses/balanced_cross_entropy.py

bob/learn/tensorflow/losses/pixel_wise.py

bob/learn/tensorflow/metrics/__init__.py

bob/learn/tensorflow/metrics/embedding_accuracy.py

bob/learn/tensorflow/metrics/pixel_wise.py

bob/learn/tensorflow/models/__init__.py

bob/learn/tensorflow/models/densenet.py

bob/learn/tensorflow/utils/image.py

bob/learn/tensorflow/utils/math.py

doc/nitpick-exceptions.txt

doc/py_api.rst