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. 


This will just infinitely yield shuffled trees? That seems weird. I think this should either return a single shuffled tree, or  yield ``n`` shuffled trees. What is the motivation for the current behavior? It seems like a niche case (that would be easy to achieve when you want it) so I don't think it should be built into the method.