Move data_mean into data_transformer

Move data_mean into the data_transform class to facilitate data transformation. 
This way Data layers don't need to hold it and pass it all the time. 

Because at time of construction it doesn't know the size needed for the mean. It is resized dynamically if needed, but I think it should be removed because that case is when the mean is 0.

Look at #569 https://github.com/BVLC/caffe/pull/569/files#diff-d9395f16f374961dae87b794d924b091R16