added plot.cladeprobs function

this is really just a clone of the current plawty function, but a bit
more efficient in how it melts the input data. I also changed the look
of the plot, and made it default to all clades, just to see what it
looks like.
added plot.cladevar function

this plots the absolute variation in posterior probabilities over time.
I.e. for each clade that's seen, we want to know how much it comes and
goes. I think there's a better way to do this, but it's a rough proof
of principle right now.