Style transfer

Implemented style transfer strategy from

Gatys, Leon A., Alexander S. Ecker, and Matthias Bethge. "A neural algorithm of artistic style." arXiv prepri nt arXiv:1508.06576 (2015)

Some stuff are missing still:

• Fix a bug in Inception V1 with batch norm variables
• Enable several images to encode the style (today is only one)
• Think on how can we abstract normalization operations
• Test unit