Modified mvn_layer.cpp

I have tested the mean-variance normalization in https://github.com/zszhong/caffe/tree/test/test/test_mvn. But it seems to have different results from that outputed by `https://github.com/zszhong/caffe/tree/test/test/test_mvn.m` with the same input data.

The four types of normalization have been tested:
        1. mean normalization per channel
        2. mean normalization across channels
        3. mean-variance normalization per channel
        4. mean-variance normalization across channels

The `type 1` and `type 3` have no problem with the original code. But the result of `type 2` and `type 4` seems to be different from that in `MATLAB`. The fixed code has provide the same results as in `MATLB`.