Redesign the data shuffler

bob/learn/tensorflow/datashuffler/Base.py

bob/learn/tensorflow/datashuffler/Disk.py

bob/learn/tensorflow/datashuffler/Memory.py

bob/learn/tensorflow/datashuffler/Siamese.py

bob/learn/tensorflow/datashuffler/SiameseDisk.py

bob/learn/tensorflow/datashuffler/SiameseMemory.py

bob/learn/tensorflow/datashuffler/Triplet.py

bob/learn/tensorflow/datashuffler/TripletDisk.py

bob/learn/tensorflow/datashuffler/TripletMemory.py

bob/learn/tensorflow/datashuffler/__init__.py

bob/learn/tensorflow/script/train_mnist.py

bob/learn/tensorflow/trainers/Trainer.py