traceplot allows for MutliTrace traces to be plotted separately

In the current master, `traceplot` concatenates the traces in a `MultiTrace` object into a single long chain before plotting them. Often, we will want to plot each chain separately on the same axes. I have added a `combined` argument that defaults to False, so that we get the following:

![unknown-2](https://f.cloud.github.com/assets/81476/1569144/85654770-50be-11e3-9987-df482cfa563f.png)

Setting `combined=True` will result in concatenation before plotting.

I also cleaned up the Gelman-Rubin diagnostic code while chasing a bug.