Plot Method

[lukesonnet]

Add a plot method for rollcalls. The difficulty with a plot method is that it requires both member nominate data and roll call nominate data. The options are:

1. Take a rollcall id and build the plot by pulling everything from the web.
2. Take a rollcall id and a rollcall object and plot using all of the nominate data stored within. Drawing the ellipse may require more data than is currently downloaded.