Tree method addition

This adds the functionality of cogents ```totalDescendingBranchlength()``` method to skbio ```TreeNode```. 

There are two components of the code that I am unsure about. 
First is the copying of the tree that occurs on line 2536. I didn't want to propagate my own attribute through self so I copied the tree. Is there a superior way to do this?
Second, I am moving through the tree several times. For instance, I move through to remove the internal node names to avoid a ```DuplicateNodeError```. I could wrap this in to another one of the traversals but unsure if necessary. 

Thanks to @wasade for the main logic of this function.