Implement ONNX export

[ogrisel]

https://github.com/onnx/onnxmltools has exports for lightgbm and scikit-learn decision trees.

It would be great to add onnx exports built-in pygbm, for instance in a new pygbm.onnx module (that is not imported by default so as to make the dependency on the onnx package optional).

This would also make it possible to improve the checks in test_compare_lightgbm so as to check that the structure of the generated trees are the same.

[ogrisel]

Ping @xadupre if you are bored with scikit-learn :)