modified node.py, tree.py and tests/test_treelib.py :

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
modified tree.py :

o improved Tree.paths_to_leaves() following caesar0301's suggestion
o sorted functions in tree.py by alphabetical order
Merge pull request #30 from suizokukan/master

Node.is_root() and Tree.paths_to_leaves()