[fix] Fix detected objects in extract_features_vmb (facebookresearch#159

)

* [fix] Correctly ignore the background class

* [fix] Output the correct number of objects

[feat] Add script for feature extraction from vmb (facebookresearch#93)

* [feat] Add script for feature extraction from vmb

* [fix] Address comments in the PR

* [fix] Address Meet's comments

* [fix] Remove os.path.exists from download_file

v0.1

VQA challenge 2018

This release contains reference for the code used to win VQA Challenge
2018. Checkout to this tag and follow instructions in README to
reproduce results.