simplified setup routine

[dominikandreas]

most people who already have the latest torchvision, torch, apex and pycocotools installed can now just do
pip install git+https://github.com/facebookresearch/maskrcnn-benchmark

[botcs]

Hey @dominikandreas,
This PRs seems to be an awesome idea.

I'm not so familiar with the version tags, but I think if somebody is only using the lib for demo and so, probably he would aim for a stable version, like v0.1.
Do you know how to pip install from github with specific version tags?

Thanks,
Csabi