Sort by geo distance and small fix to geo distance filter

Hi Alexander,

I've added sort by geo distance. I decided to go with creating new overloads of the Field method, which seemed to fit with the design. This might make the Field method a little too overloaded?

I've also fixed a small bug with the geo distance filter. I needed the filter to be omitted from the query when a null distance parameter is supplied.   

Can you review my code please and let me know what you think?

Thanks,
Mike