New session management

bob/learn/__init__.py

bob/learn/tensorflow/__init__.py

bob/learn/tensorflow/layers/AveragePooling.py

bob/learn/tensorflow/layers/InputLayer.py

bob/learn/tensorflow/layers/MaxPooling.py

bob/learn/tensorflow/loss/ContrastiveLoss.py

bob/learn/tensorflow/loss/TripletLoss.py

bob/learn/tensorflow/network/SequenceNetwork.py

bob/learn/tensorflow/test/test_cnn_scratch.py

bob/learn/tensorflow/trainers/Trainer.py

bob/learn/tensorflow/utils/__init__.py

bob/learn/tensorflow/utils/session.py

bob/learn/tensorflow/utils/singleton.py

bob/learn/tensorflow/utils/util.py

doc/user_guide.rst