Add ST_Rotate

Note: By default, PostGIS rotates a geometry about POINT(0,0), while
GDMS rotated the geometry about the center of its internal envelope.

TODO: Allow the user to rotate a geometry about an arbitrary point, via
        ST_Rotate(Geometry geom, double theta, double x0, double y0)
        ST_Rotate(Geometry geom, double theta, Geometry pointOrigin)