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.
*keyed* and *valued* here make it sound like a ``dict``. Could you say something like: *where the first element in each tuple is the `frozenset` of the clade, and the second element is the support value. It is expected that this list is sorted by descending support.*

(Note typo in your current second sentence: *sorted by in*.)

Also,