ENH: Improved Circle Plot

Some circle plot improvements to make it work in subplots, also exposes additional options for font sizes etc.

There may be a bug in matplotlib; it doesn't take into account the sizes of the node labels when arranging the subplots, so they end up overlapping. As a workaround I added the `padding` parameter which allows the user to manually add space around the circle. The second plot from the example looks like this:

![circle](https://f.cloud.github.com/assets/647005/1979287/bf046c94-83af-11e3-9b83-1886fc0c89dd.png)


I wanted to keep the first figure as it has 1:1 aspect ratio, which looks nice in the thumbnail page.