Bug fix for describe.simmap

There was a small bug in this new function to summarize the results of stochastic mapping that I posted yesterday, describe.simmap, for describe.simmap(...,plot=TRUE). The fixed version of this, plus updated code for the re-rooting method of ancestral state reconstruction, are on the phytools webpage. I also posted a new minor build of phytools (phytools 0.2-31) containing these updates.