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.
Which is weird, because might expect it to be scaled. But this matches [Phylip](http://evolution.genetics.washington.edu/phylip/doc/consense.html). Could scale by the number of trees, however, that is misleading if there are multiple returned trees as the support value would need to be scaled by the trees it was observed in IMO and would realistically add a fair bit of complexity into the code in order to manage bookkeeping.