Support 3D input data (2D images with RGB channels)

[beomyeol]

The pull request #169 implements pooling layers for dophin-dnn. It supports only 2D input data. Even though this works well for MNIST dataset that consists of grayscale images, it doesn't handle RGB image datasets such as CIFAR-10 and ImageNet datasets. We should makePoolingLayer` support 3D input data (e.g. 2D image inputs with RGB channels)

[beomyeol]

Closed via #175.