Batched demo & several compatibility fixes

[Lotayou]

Hi akanazawa, thanks for the great work. I created a batched demo version that loops over an image folder, hope you find it useful.

In addition, two bugs are fixed:

• In src/tf_smpl/batch_smpl.py, fixed pickle encoding parameter error for Python 2 compatibility.
• In src/util/openpose.py, provide support for openpose v1.2+ compatiblity.

To validate my pull request, you can run

python -m demo_batch --img_path data/small --json_path data/small_keypoints_18 --result_path results

Note: tqdm is required for monitoring progress.