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.
I think this would be more clear as: *Any clade that has `support` <= `cutoff` will be dropped.*

and:

*ties are arbitrarily broken.*

Are ties broken in the same way all the time, or at random? Maybe be worth noting in the Notes section?