Cube extension point support // GSoC'13

Hello.

here is a patch adding to cube extension support for compressed representation of point cubes. If cube is a point, i.e. has coincident lower left and upper right corners, than only one corner is stored. First bit of the cube header indicates whether the cube is point or not. Few moments:

* Patch preserves binary compatibility with old indices
* All functions that create cubes from user input, check whether it is a point or not
* All internal functions that can return cubes takes care of all cases where a cube might become a point
* Added tests for checking correct point behavior

Also this patch includes adapted Alexander Korotkov's patch with kNN-based ordering operator, which he wrote for postgresql-9.0beta1 with knngist patch. More info there http://www.postgresql.org/message-id/AANLkTimhFaq6hCibRnk0tlcQMIyhYWHwAQ2ZD87wbH86@mail.gmail.com

Stas Kelvich