Handle MKAnnotations

[nicked]

Allow NAAnnotations to be created from MKAnnotations. Even better, make it a sub-protocol of MKAnnotation so I can use my existing map points.

[dblock]

With 3.0 you can easily make an NAAnnotation out of an MKAnnotation, however I think it would be good if one could just take an MKAnnotation and add it to an NAMapView, as suggested. This just means extracting a protocol out of an NAAnnotation, which is no longer a big deal, that class is extremely minimal now.