added traversal_paths function for ClusterNode class

This function can help get the path for each ClusterNode object in a hierarchical clustered object. Potential usage:

```python
import numpy as np
import scipy.cluster.hierarchy as sch

X = np.random.randn(10,10)
d = dist.pdist(X)
D = dist.squareform(d)
Y = sch.linkage(D)
Tree = sch.to_tree(Y)
leaves = Tree.pre_order(lambda x: x.id)
paths = Tree.traversal_paths() ## 
```