Different image standardizations for 2017 and 2018 models

[zhenglilei]

Thanks for this useful tool for converting facenet-tensorflow to facenet-caffe!

About the models, an important factor may have been ignored when do inference.
20170512-110547: per_image_standardization (the prewhiten function in your codes)
20180402-114759: fixed_standardization

With the correct standardization used, the 2018 model shall perform better than the 2017 model.

I also put this notice at the original FaceNet issue pool: davidsandberg/facenet#948 (comment)

[taylorlu]

Good job, I'll take some time to look into it.