merge
Add st_ringbuffer function
Add missing st_ringbuffer function registration
Fix header
Merge pull request #241 from ebocher/ringBuffer

 Ringbuffer function #241 
Add the ST_RingBuffer function.

The signature is :

```mysql
SELECT ST_RingBuffer('POINT(10 10)'::GEOMETRY, 10, 3)

```

returns a multipolygon with 3 rings buffer. The distance between each rings is 10 meters.