Setup package

[louismartin]

Setup XLM as a package.
I only checked that the imports work fine in the notebook, it should be further tested.

• Add setup.py
• Rename src to xlm
• Refactor "from src import ..." to "from xlm import ..." (ag "from src" -l | xargs perl -pe 's,from src,from xlm,g' -i)
• Add src->xlm symlink for backward compatibility

[louismartin]

@glample any plan on merging that? :p

[jrapin]

@glample any plan on merging that? :p

That would be helpful for me too, so that we can install xlm in a project instead of manually linking to src!