MNIST and CIFAR10 

1. Added various utility functions to dataset/init.lua for 
1.1 splitting a dataset into a training / validation set (dataset.splitter)
1.2 splitting a dataset into its respective classes (dataset.split_by_class)
1.3 creating a new dataset containing only examples of a given class (dataset.include_by_class)

2. Fixed CIFAR-10 loading script (bug fixes)

3. Code cleanup in MNIST loading scripts.

4. Added utility functions TableDataset:[data,class,metadata]