(libtensorflow_inference.so still required)
*   Added `categorical_column_with_vocabulary_file`.
*   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.