Feed the ImageDataLayer with OpenCV images directly from memory

The discussion of #196 led to the conclusion that the ImagesLayer (#120) by @sguada is more suitable for ingesting raw images. Following @Yangqing's [suggestion](https://github.com/BVLC/caffe/pull/196#issuecomment-37707842), this PR extends ImagesLayer to accept OpenCV format images which are used by computer vision researchers and engineers very commonly.