*   Make graph name scope thread local so that they work correctly in
        multi-threaded environments.
    *   Update nsync synchronization library to avoid slow primitives on Linux.
    *   Removed need to put nsync/public on C include path when building custom
        ops.
    *   Add `tf.image.psnr`, `tf.image.ssim`, `tf.image.ssim_multiscale`,
        `tf.image.image_gradients`, `tf.image.sobel_edges`.