Major change to functionality of randomlyAddTaxa

Two major changes to function, plus a minor change. Minor change: added better example. Major change 1: tree is now built up iteratively, so that new species can be bound to either a species in the original phylogeny, or to a species that was added in subsequently. Major change 2: can now choose to add species either above, below, in a polytomy or randomly above or below the node in question.
Fixed bug: adding taxa stem-wards

There is a check built into this function that if a single taxon is sister to all others, and if "stem" or "randomly" is chosen, any taxa to be added will always be added crown-wards to avoid function failure by trying to add the taxa below the root. Fixed incorrect calculation of whether the first part of this if statement was true.
Merge pull request #19 from eliotmiller/patch-3

Major change to functionality of randomlyAddTaxa