add minDistance option to $near geospatial operator

[eleyvas]

$minDistance limits the results to those documents that are at least the specified distance from the center point. $minDistance is only available for use with 2dsphere index.

[evanchooly]

I think this should cover this case. The geo API is one of my least favorites as it's an area I don't really know well (geospatial work, that is). It's on my radar as one of the areas that could use a lot of housecleaning going forward. any input would greatly appreciated.