Implement the importing of all layers

[moazreyad]

Currently the converter only imports the layers of the AlexNet example. (MaxPool, Conv, Relu, Add, Reshape, Dropout, Gemm)

After testing this example and making sure these layers are converted successfully (see #1) , we need to proceed by supporting also the rest of the onnx layer types.

These layers/nodes include: slice, concat, pad, average pool