Plot enhancements

Made some updates to plotMap(). Now we return the plot object rather than the figure number.
The figure number is still available, if we wanted to return it, too, but I think this is more useful.

The FramedPlot object can be used to save the plot with the correct aspect ratio in any form that Winston supports. The figure number can be used to programmatically close a figure, and can only save figures using a default, square aspect ratio in the preset size (as far as I can tell). I find it more valuable to be able to save higher quality map images.

Also added some more minor plotting features, which are disabled by default. You can plot in kilometers and fix the font size of the labels. These are all things I realized I needed for a paper I'm working on, in order to have consistency amongst all my figures.

Returning the plot object also allows more in-depth tests of plotting functionality, even though we can't actually "see" the plot.