[Tile38] Add ability to search object nearby other objects (linestring buffering)

Currently the NEARBY command only supports a radius search around single a point, such as: ``` NEARBY poi POINT 41.2801 -7.7214 100 ``` But it would be great to add the ability to search nearby other objects such as: ``` NEARBY poi OBJECT {"type":"LineString","coordinates":[[-7.71175,41.28433],[-7.71708,41.28233],[-7.72145,41.28012],[-7.72579,41.27761],[-7.72791,41.27710],[-7.73216,41.27718],[-7.74237,41.27782]]} 100 ``` Under the hood the NEARBY command would need to generate a new polygon from the linestring by "buffering" the linestring using an algorithm like the [Minkowski Sum](https://en.wikipedia.org/wiki/Minkowski_addition), [PostGIS ST_MinkowskiSum](http://postgis.net/docs/ST_MinkowskiSum.html). Original LineString:  Buffered LineString: ![LineString-Buffer](https://cloud.githubusercontent.com/assets/1156077/20146028/82e44fd8-a65f-11e6-