Introduce SP-GiST operator class over box.

Patch implements quad-tree over boxes, naive approach of 2D quad tree will not
work for any non-point objects because splitting space on node is not
efficient. The idea of pathc is treating 2D boxes as 4D points, so,
object will not overlap (in 4D space).

The performance tests reveal that this technique especially beneficial
with too much overlapping objects, so called "spaghetti data".

Author: Alexander Lebedev with editorization by Emre Hasegeli and me