[DO NOT MERGE] Make object_detection pip installable

[hadim]

DO NOT MERGE

Following this issue #4887, I did a small test and everything seems to be ok to make the object_detection pip installable.

Protobuf files compilation works during pip installation (with or without the -e option): https://github.com/hadim/models/blob/535affae95d5da46b8819989cb6e1123d169d245/research/object_detection/setup.py#L18

The PR is pretty big because I add to move the Python files into a subdirectory called object_detection/.

I have also added an entry point for model_builder_test. Of course many more can be added if needed.

Note also the addition of an environment.yml file making the installation of deps much easier. People don't have to worry anymore about installing protobuf since it's available on Anaconda.

Important: at the moment this PR breaks the object_detection API code and model_builder_test returns an error when it is run. This is because the slim package provided by tensorflow seems to be older than the one provided by this repo. I don't have for the moment a good solution for this. I can suggest two options:

1. Update the Tensorflow slim API to match the one provided on this repo.
2. Make the slim package provided by this repo "official" and release it on PyPi. Then it's just a matter of adding slim as a pip dependency.

This PR is mainly for demonstration purpose.

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[hadim]

I signed it!

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[tfboyd]

6+ months old. Cleaning up a few PRs that show up if I filter for /official. While I do not think this touches official, closing this seemed like helpful cleanup. Ping me to reopen of desired. It was also tagged DO NOT MERGE in the title so I do not feel closing if going to offend you.

[hadim]

It does not offend me but that would be nice to consider/discuss on #4887.