Support for circles that cover the poles

Circles that cover the poles aren’t rendered correctly right now, as shown here:
![Incorrect Polygon](https://cloud.githubusercontent.com/assets/614874/3708000/e83da6fc-143c-11e4-9d3e-ac3c8201d3ff.png)

If the angular radius from the centre is bigger than the colatitude, the polygon should cover the area of pole down to the latitude of the southernmost point. The polygon is modified by inserting four points to ensure this.
Same method is applied when the south pole is covered by the circle.

![post](https://cloud.githubusercontent.com/assets/614874/3708006/f455f7fa-143c-11e4-92a3-3d4c5d8659f8.png)

The “width” is limited to 360 degrees so that the correct area of the polygon can be calculated.
Making the width bigger by adding/subtracting more than 180 would look nicer, but causes new errors (e.g. missing repetition of the “curved part”)