Add weights of VGG/reset models trained on Open Images

[gabrieldemarmiesse]

Currently, for all models, we can do weights='imagenet' or weights=None. We would like to add weights='openimages'. From @taehoonlee in #26 :

I think that it is worth and significant differentiation to add reference models trained with Open Images. Contributions welcome :) In my humble opinion,

Using different datasets without hyper-parameter changes: it is easy and worth to add because we just need to replace weights='imagenet' with weights='openimages', classes=5000 and add information on weight files. Using different datasets with minor hyper-parameter changes (e.g., changing the numbers of blocks in ResNets): if factorization over imagenet and openimages models is easy, it is still worth to add keras-applications. Using different datasets with significantly different hyper-parameters (e.g., changing the block configuration or adding some layers): it is better to consider them as the community contribution and exist outside of keras-applications.