Rtree with backing storage

This implements bounding box queries using a RTree with STR-Packing. The RTree uses an external backing store to minimize RAM usage.

The query time for nodes seems to be rather high (4Mio Nodes, 60ms to return all nodes on zoom level 0), I'm not exactly sure if that indicates a bug, or is just caused by hard disk I/O.

Please test this on multiple platforms / data set sizes.