Issue422

Sorry for the limited granularity on the commit history. This adds in majority rule and addresses #422. It should perform fairly better than the cogent implementation, but not awesomely better. I'm pretty certain that the lookups into `accepted_clades` can be optimized so that, instead of a O(N) search through a list that a `dict` could be used to provide O(1)-ish lookups to ancestors. I was planning on benching that tomorrow, but was hoping to get a review in parallel while finalizing performance tweaks.
Right, but it was tested with multiple inputs by varying weights and the decomposed internals used different trees the the driver method. I don't disagree in principle, but I think that is redundant in this case. Regardless, there is a second test in place now that covers the multiple trees.