Add two functions to create an elliptical arc as line or polygon.

[ebocher]

This PR adds two function to create an elliptical arc as line or polygon from a given point, a distance, a start angle and the size of it in radians.

Fix ST_MakeEllipse to keep the srid of the centre point

Exemple :

SELECT ST_ACCUM(the_geom) from (SELECT ST_MakeArcPolygon('POINT (0 0)'::GEOMETRY,  X*10, 
                CASE WHEN X=1 THEN 0 ELSE PI()/X END, PI()/X+1) as the_geom 
                FROM generate_series(1, 3)) as foo