Fix map background

I have added an example but need Jonathan to run doc generation on it.. Uses NSA CF file from ASR tutorial as example.. 

A few enhancements to PPI plotting with map backgrounds, specifically cross hairs, labels and range rings.. Underlying routines can also be used to plot things like aircraft tracks or other shapes.. 
![nsa](https://f.cloud.github.com/assets/825351/2503974/118b43c2-b388-11e3-85e4-93562ec0e6a3.png)