Packaging fix

[davidwilby]

Hey, sorry another one from me.

I realised that the way setup.py was structured before didn't actually work because of the directory arrangement of NMT-Keras.

This version now definitely works and makes NMT-Keras an importable python package, eg things like

from nmt_keras.utils.utils import update_parameters

now work.

If you want to include any more non-python parts of the repo in the python package, then they will need to be added to package_data in the setup.py.

[lvapeab]

Thanks @davidwilby! I fixed the pep 8 style. Eventually, we'll need to add tests for this. But for the moment seems good to go.