KNN searches support for SP-GiST [GSOC'14]

Hi there, pg-Hackers!
Here I go with the patch which brings up the possibility to perform
nearest-neighbour searches on SP-GiSTs (as of now includes implementation
for quad and kd trees). Pre-reviewed by my GSoC mentor Alexander Korotkov.
Sample benchmarking script included in the attachment (dumps the current
geonames archive and runs several searches on the (latitude, longitude)
points), which demonstrates the dramatic improvements against plain
searches and sorting. Regression tests included, compiles and runs
successfully under both of my Ubuntu 12.04 Server and 08/2014 Arch Linux.

Vlad Sterzhanov / Quadrocube