Merge pull request #29 from suizokukan/master

modified node.py and tree.py : improved documentation
Remove unsed NodeIDTypeException.
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