Tranform layers [DON'T MERGE]

This is a tentative approach to add transformation layers that would allow to crop_mirror, center_scale the blobs.

The first use would be to help Data source and pre-processing separation. The source would produce a vector of blobs, one per image, the transformation layers and finally concat all the blobs into one.

The concept is similar to do `map(transformation, bottom_blobs, top_blobs)` 

@Yangqing What do you think?