Improve st_buffer signature.

The user can now set specific buffer style parameters.

Supported signatures are

```mysql
SELECT ST_BUFFER('POINT(10 10)'::GEOMETRY, 20, 'quad_segs=8 endcap=round|flat|square join=round|mitre|bevel mitre_limit=5');
```

or

```mysql
SELECT ST_BUFFER('POINT(10 10)'::GEOMETRY, 20, 2);
```

It's similar to http://www.postgis.org/docs/ST_Buffer.html.