KML writer function

This PR adds two new function : KMLWrite and ST_AsKML

The signature of the KMLWrite driver function is

```mysql
CREATE TABLE KML_POINTS(id int primary key, the_geom POINT, response boolean);
INSERT INTO KML_POINTS VALUES(1, ST_Geomfromtext('POINT (2.19 47.58)', 4326), true);
INSERT INTO KML_POINTS VALUES(2, ST_Geomfromtext('POINT (1.06 47.59)',  4326), false);
 // Write the KML file
CALL KMLWrite('.../kml_points.kml', 'KML_POINTS');
```

The ST_AsKML function could be used to write KML geometries.

```mysql
CREATE TABLE KML_POINTS(id int primary key, the_geom POINT, response boolean);
INSERT INTO KML_POINTS VALUES(1, ST_Geomfromtext('POINT (2.19 47.58)',4326), true);
// Write a KML geometry
 SELECT ST_AsKml(the_geom) from KML_POINTS;
```
 returns : 

```xml
<Point><coordinates>2.19,47.58</coordinates></Point>
```
This function supports two arguments : extrude (boolean) and altitude mode (integer).
Available extrude values are true, false or none.
Supported altitude mode :
 * For KML profil : CLAMPTOGROUND = 1; RELATIVETOGROUND = 2; ABSOLUTE = 4;
 * For GX profil : CLAMPTOSEAFLOOR = 8; RELATIVETOSEAFLOOR = 16; 
 * No altitude : NONE = 0;

Note : that I have no time to code a KML reader but it will be useful to do that in the future.