Add ST_MakePoint

See #41.

### Example usage

```mysql
SELECT ST_MakePoint(1.4, -3.7);
```
Answer:     `POINT(1.4 -3.7)`
```mysql
SELECT ST_MakePoint(1.4, -3.7, 6.2);
```
Answer:     `POINT(1.4 -3.7 6.2)`

*Note*: PostGIS has a fourth signature for geometries with measure:
```mysql
    geometry ST_MakePoint(double precision x, double precision y,
        double precision z, double precision m);
```

See http://postgis.net/docs/manual-1.4/ST_MakePoint.html