Add plot() method to the aperture classes

This PR adds a plot() method to the aperture classes.  

~~The `CircularAnnulus` and `EllipticalAnnulus` apertures do not yet support fills.  I am working on creating custom patches to allow that, but this PR can be merged now if needed for the examples in the documentation.~~

Update:  I've made custom apertures for the annuli and this should be ready to merge.