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. 


Was torn on that. This method, `nj`, the creation methods mentioned in the PR and other methods could arguably go under `TreeNode` but at some point we just blow up the namespace under that object. I don't know how best to make the distinction right now between what should be member methods and what should just be functions.