*   Introduce ops for batching/unbatching tensors across Session::Run() calls.
*   Add tf.log_sigmoid(x) = tf.log(tf.sigmoid(x)) = -tf.nn.softplus(-x).
*   Changed hooks lists to immutable tuples, and now allow any iterable for the
    associated arguments.
*   Introduce TFDecorator.
*   Added an Mfcc op for speech feature generation.
*   Improved DirectSession::Run() overhead and error checking. Feeding a value