Make the object_detection folder pip installable

Please see https://github.com/tensorflow/models/issues/917#issuecomment-407552473 for history.

In short, that would allow to really use object_detection as a lib and an API without having to clone the whole repository.

It would make it more easy to use and everyone's code using this API more reproducible.

Now it's up to you whether you want to keep it in this repo or create a separate repo from models. The advantage of the later is that you can easily install whatever git commit you to want with something like that: pip install https://github.com/tensorflow/models/archive/6c21084503b27a9ab118e1db25f79957d5ef540b.zip.

That being said I am totally fine with regularly upload wheels to PyPi from only the research/object_detection folder.

The last thing for a pip installable package, you would have to compile PB files in the setup.py file, but that should not be too hard. I can submit a PR if you want/need.

Hope this makes sense to you.