Fixed issues with building NMS and running demo

[romamartyanov]

Fixes provide by pull request #38 of @IAmSuyogJadhav :

• Fixes Error when running demo #22, also added instructions in the README.
• Fixed Bounding Boxes displacement error #15, Applied the necessary changes directly in the code.
  Added instructions in the README to prevent ConnectionRefusedError #27 (ConnectionRefusedError)

Also his pull request Fixes #39.

New fixes provided by @romamartyanov:

• Fixes python build.py build_ext --inplace error #53 by changing _nms_gpu_post.c file with the help of this reply
• Fixes issue related to the call of decorator from the new version of Cython in file non_maximum_suppression.py