Add specialized .NET-Bindings to the TensorFlow repo

[henon]

This is an RFC that will be posted in TensorFlow community as suggested at

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Hello TensorFlow Team,

for the development of TensorFlow.NET we need some special wrappers to the C++ core which are of course different from the Python wrappers. We think that our wrappers should be included in the Tensorflow repo in a subfolder called "csharp". To start the conversation, we made a PR which was declined with the suggestion to discuss it here first.

This is what the csharp binding will (or will not) contain:

• it will complete most of the key functions and act like the python binding but for c#.
• it will add more custimized APIs in the cswrap_tfe.h, making changes in the seperate folder will reduce the main repo's risk.
• it will contain only C++ exported functions in tensorflow repo. The C# code will still remain in the tensorflow.net repo.

The reasons for this suggestion

• There are already c, cc, java, js, pyton and go bindings included in the tensorflow repo. We'd like for csharp to be included as well.
• TensorFlow.NET is a widely popular library with 6 million potential developers in the .NET community. We think that warrants recognition and inclusion of the necessary csharp bindings
• We don't ask you to develop the bindings. TF.NET committers will take that responsibility.

[Oceania2018]

As @MarkDaoust mentioned, it might be a better idea to add SIG first. https://github.com/tensorflow/community/tree/master/sigs

[Oceania2018]

Added PR.