Supported alphashape hole polygon case

This fix will support alpha shape hole polygon case (#235).
And I confirmed that current all test passed on MacOSX.

I changed pgr_alphaShape/pgr_pointsAsPolygon specification as follows.
* pgr_alphaShape - Returns multiple outer/inner ring coordinates with separator row (x=NULL and y=NULL)
* pgr_pointsAsPolygon - Returns multiple polygons (with hole)
In case of single non-hole polygon, the result will be same shape as current.
(But coordinates' order is different a bit.)

I hope someone review my modification.