Writing user guide and fixed warnings

bob/learn/tensorflow/configs/style_transfer/vgg19_example.py

bob/learn/tensorflow/dataset/__init__.py

bob/learn/tensorflow/estimators/Logits.py

bob/learn/tensorflow/estimators/Siamese.py

bob/learn/tensorflow/estimators/Triplet.py

bob/learn/tensorflow/estimators/__init__.py

bob/learn/tensorflow/network/__init__.py

bob/learn/tensorflow/utils/util.py

doc/img/angelina.jpg

doc/img/angelina_output.jpg

doc/img/vincent_van_gogh.jpg

doc/py_api.rst

doc/user_guide.rst