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. 


I totally get where you are coming from, however we aren't necessarily the intended audience here. Fundamentally this is a package for biologists, not programmers or computer scientists. I think a generator factory is more in line with what a user will expect. Someone will certainly attempt a situation that results in an inexplicable (to them) infinite loop.

Furthermore it is usually better to be explicit (i.e. using your `shuffle(iter=inf)` which is a great idea) rather than implicit, and we can save the user a couple keystrokes by making this a factory, which is usually our job.