`reduce_` ops.
*   Move `TensorForestEstimator` to `contrib/tensor_forest`.
*   Enable compiler optimizations by default and allow configuration in
    configure.
*   `tf.divide` now honors the name field.
*   Make metrics weight broadcasting more strict.
*   Add new queue-like `StagingArea` and new ops: `stage` and `unstage`.
*   Enable inplace update ops for strings on CPU. Speed up string concat.