Implemented the prefetch for the siamese trainer

bob/learn/tensorflow/analyzers/Analizer.py

bob/learn/tensorflow/data/BaseDataShuffler.py

bob/learn/tensorflow/data/MemoryDataShuffler.py

bob/learn/tensorflow/data/MemoryPairDataShuffler.py

bob/learn/tensorflow/data/TextDataShuffler.py

bob/learn/tensorflow/data/TextPairDataShuffler.py

bob/learn/tensorflow/data/__init__.py

bob/learn/tensorflow/data/copy of PairDataShuffler.py

bob/learn/tensorflow/script/train_mnist.py

bob/learn/tensorflow/script/train_mnist_siamese.py

bob/learn/tensorflow/trainers/SiameseTrainer.py

bob/learn/tensorflow/trainers/Trainer.py

bob/learn/tensorflow/util.py