Add ST_InteriorPoint

Note: PostGIS calls this ST_PointOnSurface. See also
ST_Point_Inside_Circle.
Add ST_PointsToLine

Note: This is called ST_MakeLine in PostGIS. There are three versions:

    1. geometry ST_MakeLine(geometry set pointfield);
    2. geometry ST_MakeLine(geometry point1, geometry point2);
    3. geometry ST_MakeLine(geometry[] point_array);

For now, we only have the equivalent of (1) on (Multi)Points. For
MultiPoints, PostGIS has a separate function:

    geometry ST_LineFromMultiPoint(geometry aMultiPoint);
Add ST_ToMultiPoint

Note: The only way this seems to be possible in PostGIS is via

    SELECT ST_Multi(ST_Union(the_geom)) FROM myTable
Add ST_ToMultiLine

This seems to be possible in PostGIS only by doing something like

        SELECT ST_Multi(ST_Union(the_geom)) FROM myTable;
Add ST_Holes

This looks complicated in PostGIS. Something like:

    SELECT AsText(InteriorRingN(
        GeometryN(geom, gn.n),
        NumInteriorRings(GeometryN(geom, gn.n))))
            AS geometryOfHole FROM areas,
        generate_series(1, 1000) gn(n)
            WHERE NumGeometries(geom) >= gn.n
            AND NumInteriorRings(GeometryN(geom,gn.n)) > 0

See here:
http://lists.osgeo.org/pipermail/postgis-users/2007-August/016829.html
Add ST_ToMultiSegments

This seems to be complicated in PostGIS. See
http://stackoverflow.com/questions/7595635/how-to-convert-polygon-data-into-line-segments-using-postgis
Remove ST_InteriorPoint

This SFS function was already implemented in h2spatial.
Merge pull request #52 from agouge/convert-fns

Here we add the following convert functions:

* `ST_Holes`
* `ST_PointsToLine`
* `ST_ToMultiLine`
* `ST_ToMultiPoint`
* `ST_ToMultiSegments`

#### Comparison with PostGIS

* Add `ST_Holes`

This looks complicated in PostGIS. Something like:
```mysql
SELECT AsText(InteriorRingN(
    GeometryN(geom, gn.n),
    NumInteriorRings(GeometryN(geom, gn.n))))
        AS geometryOfHole FROM areas,
    generate_series(1, 1000) gn(n)
        WHERE NumGeometries(geom) >= gn.n
        AND NumInteriorRings(GeometryN(geom,gn.n)) > 0
```

See here:
http://lists.osgeo.org/pipermail/postgis-users/2007-August/016829.html

* Add `ST_PointsToLine`

*Note*: This is called `ST_MakeLine` in PostGIS. There are three versions:
```java
geometry ST_MakeLine(geometry set pointfield);
geometry ST_MakeLine(geometry point1, geometry point2);
geometry ST_MakeLine(geometry[] point_array);
```

For now, we only have the equivalent of the first form on (Multi)Points.
For MultiPoints, PostGIS has a separate function:
```java
geometry ST_LineFromMultiPoint(geometry aMultiPoint);
```

* Add `ST_ToMultiLine`

This seems to be possible in PostGIS only by doing something like
```mysql
SELECT ST_Multi(ST_Union(the_geom)) FROM myTable;
```

* Add `ST_ToMultiPoint`

*Note*: The only way this seems to be possible in PostGIS is via
```mysql
SELECT ST_Multi(ST_Union(the_geom)) FROM myTable
```

* Add `ST_ToMultiSegments`

This seems to be complicated in PostGIS. See
http://stackoverflow.com/questions/7595635/how-to-convert-polygon-data-into-line-segments-using-postgis

See #40.