Add pybind11

[lioulirong]

What is this Python project?

• pybind11 is a lightweight header-only library that exposes C++ types in Python and vice versa, mainly to create Python bindings of existing C++ code. (cite from the official doc)

• Inspired by Boost.Python, but aim to resolve the complexity.

• A very compact implementation. It's only composed of ~4K line without comments.

What's the difference between this Python project and similar ones?

1. It's a lightweight, header only C++ library.
2. It supprots modern C++ (instead of just C)
3. One need not to learn a third language other than C++ and Python. (Some others require)
4. Also Used by popular project like Cafe2 machine learning framework.
5. Its documentation is quite complete!

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