add labels for lexical and synset nodes

I made this branch for the node labels issue.

I'll take this opportunity to ask some questions : 

* if I want to work on an other task should I branch again ( from node_labels ? master ? ) 
* I sometimes think about some improvements ( like possibility to add as much as labels as we need without touching directly writing functions, create a folder for each language in csv_files, etc ... ) should I open a issue for each or a big one for all ?