Update ilsvrc_2012_mean.mat to W x H x C, update demo and add comments

Update ilsvrc_2012_mean.mat to W x H x C, update demo and add comments

Update previously ilsvrc_2012_mean.mat stores 'image_mean' variable in
H x W x C with BGR channels, which is inconsistent with Caffe's data format
and inconsistent with caffe.io.read_mean(..). Replace 'image_mean' with
'mean_data' variable in W x H x C and update classification_demo.m. Also add
some comments.