mean-variance normalization layer

Normalizes activations to zero-mean unit-variance for each channel of each datum.

Unit tests included.
It's ok -- the per-channel normalization is useful alone, it's not necessary to extend to per-sample  for this PR.  I just thought you had also implemented the per-sample normalization due to your comment I quoted above?  Anyway, it's up to you whether you want to also implement the per-sample normalization (and then move to common_layers.hpp if you do); otherwise I think we can merge this once you address my `caffe_set` nitpick.