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.
Do we have permission from (or contact info for) Matthew Wakefield? Is it similar enough that we need to have permission for the port, or did you change substantially in the process? In either case, we should add him to the [list of pre-history contributors](https://github.com/biocore/scikit-bio#the-pre-history-of-scikit-bio).