Add ST_Delaunay and ST_ConstrainedDelaunay

Two new functions have been added: `ST_Delaunay` and `ST_ConstrainedDelaunay`.

`ST_Delaunay` creates a Delaunay triangulation and returns a collection of
`Geometry`s. There are three possible signatures:
```mysql
SELECT ST_Delaunay(the_geom);
SELECT ST_Delaunay(the_geom, 0);
SELECT ST_Delaunay(the_geom, 1);
```

`ST_ConstrainedDelaunay` creates a constrained Delaunay triangulation and
returns a collection of `Geometry`s. Possible signatures:
```mysql
SELECT ST_ConstrainedDelaunay(the_geom);
SELECT ST_ConstrainedDelaunay(the_geom, 0);
SELECT ST_ConstrainedDelaunay(the_geom, 1);
```

The integer parameter is used to set the output format:

* 0 to return triangles as `MULTIPOLYGON`s
* 1 to return edges as `MULTILINESTRING`s

If none is specified, the default value is 0. I.e.,
```mysql
ST_ConstrainedDelaunay(the_geom) = ST_ConstrainedDelaunay(the_geom, 0)
```
are equivalent.

*Note*: If the z-value is equal to `NaN`, the algorithm sets a 0 value ordinate.

#### Comparison with PostGIS

```mysql
ST_DelaunayTriangles(geometry g1, float tolerance, int4 flags);
```
See http://postgis.org/documentation/manual-2.1SVN/ST_DelaunayTriangles.html