[add] OneParentNode.move() to move a OneParentNode from a parent to another
Merge branch 'master' of git://github.com/titouanc/beta402

Conflicts:
	polydag/models.py
[fix/enh] A doc now may not have multiple parents. So a parent id may be used for the viewer (done).
Files are now stored in a directory  whitch name id the parent id
[fix] Merge from C4