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.

On final thing, I've added a sort array to the hit result. In my context I'm using this to return the distance between restaurants.

Can you review my code please and let me know what you think?

Thanks,
Mike