Add a new demo for estimating optical flow on a single image pair

[HuangJunJie2017]

Add a new demo for testing.
The demo show how to use this project to estimate optical flow on a single image pair, as well as how to save the result.
corresponding to the issue#51 - https://github.com/NVIDIA/flownet2-pytorch/issues/51

[christian-rauch]

In future, can we please avoid hardcoded absolute paths like /home/hjj/flownet2-master/data/FlyingChairs_examples? Paths should be determined by a parameter. I also believe that commit messages should be in English so they can be understood by a larger audience. Since this PR is only adding a single file to the repo, this should have been done in a single commit. Having this merged is not a good sign of software quality.