Improvements

[mmaaz60]

• Fixes the validation of the model on coco minival dataset
• Adds a script to download coco dataset
• Adds info in README about which environment works and which does not
• Updates requirements.txt to work seamlessly. Numpy and Cython should be installed first otherwise you will get the error.
• Fixes COCO Minival 2017 data and Annotations #6

Thanks