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. 


Good point, I'm also not sure about the distinction. This seemed pretty natural to have as a `TreeNode` method since you're applying an operation to an existing tree and the shuffling is done in-place. I don't have strong feelings either way, @gregcaporaso @ebolyen what do you think?