Update to Python 3 and other nice things

[varunagrawal]

• Formatted the entire code to work with Python3
• Ran a linter and autoformatter on the code to ensure it conforms to good coding styles for python
• Added download scripts for Images, Annotations and Questions to download the data and minimize work for the user. I am sure you can add in more functionality for other versions of the dataset.

[varunagrawal]

@AishwaryaAgrawal you might want to squash and merge this PR since I have a lot of minor commits that don't make logical sense independently.

[varunagrawal]

Cleaning up old PRs.

@deshraj @dexter1691 @deviparikh