Add in utility method for subsetting

[mortonjt]

Probably should add in utility method to handle tips in the TreeNode.subset method.

https://github.com/biocore/scikit-bio/issues/1525

So it would be something like

def fetch_tips(t):
    if len(t.children) == 0:
        return {t.name}
    else:
        return t.subset()