Node.is_root() and Tree.paths_to_leaves()

Not sure if these two functions are important but I missed them and I thought it was better to share my code. Tell me your opinion !

commit message :
o added a function Node.is_root() : return True if self is the root node.
o added a test for Node.is_root() in tests/test_treelib.py
o added a function Tree.paths_to_leaves() : return the paths of identifiers
  between the root node and each leaf.
o added a test for Tree.paths_to_leaves() in tests/test_treelib.py