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.
One other comment, jackknifing is itself a use-case for multiple trees. Jackknifing is sampling without replacement, and if performed on the sequences instead of the alignment columns, then set of tips in a given tree is != to the set of tips in another tree. I don't recall if the qiime code currently is on the sequences or the columns, or if it assumes the first tree coming back from majority consensus is the "best", but if so, I think that assumption should be revisited.