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. 


Oh, it's because checklist.py doesn't check docstrings. 

We're shooting for having everything import from the highest level import that it can to avoid confusing users by importing differently in different places, so doing a global find/replace for ``from skbio.tree import TreeNode`` to ``from skbio import TreeNode`` that would be ideal (and easy).