ENH: method to shuffle tip names on a tree

Not urgent to merge for 0.1.4, but I needed a method to help benchmark `majority_rule`, and the first one that came to mind was a way to easily shuffle tip names on a tree. I can see this method being useful in other contexts as well. 


We've been defaulting to numpy's random module, so I think ``np.random.shuffle`` would be better to use here for consistency.