Add new operators and layers

SINGA has many common operators and layers.
There are also many operators to be implemented.
Here is a list of popular operators define by ONNX https://github.com/onnx/onnx/blob/master/docs/Operators.md
Here is the list of operators implemented in SINGA http://singa.apache.org/docs/onnx/#supported-operators
The task is to add the operators in ONNX but not in SINGA.
Refer to this link for how to add new operators and corresponding layers into SINGA.