Add near,maxDistance,within to minimongo

As requested in the mailing-list : 
* $near for geospatial queries default value 25km
* $maxDistance to set a max km for $near
*  $within with $box,$circle,$polygon
* Sorting for a.b . Added this because the distance of the point to the query point is added to the object as .distance.
Only support for objects saved as {lon:0,lat:0} or {x:0,y:0}.

http://www.mongodb.org/display/DOCS/Geospatial+Indexing

    //format : [lon,lat]
    .find( { loc : { $near : [50,50] , $maxDistance : 5 } } )
    .find( { location: { $within: { $box: [[100,0], [120,100]] } } } );