Add psql native functions 'point' and 'polygon'

[kimakan]

This PR adds support for the postgreSQL native geometric functions/types point and polygon.

Now it is possible to run queries of the type

SELECT *
FROM "test_schema"."test_table" as t
WHERE POLYGON '((235.04, 24.95), (235.04, 52.90), (272.69, 52.90), (272.69, 24.95), (235.04, 24.95))' @> POINT(t.x, t.y);

The query selects all points inside the defined polygon using Cartesian coordinate system.

[coveralls]

coverage: 96.154%. remained the same
when pulling 93b5b22 on add-postgis-udf-for-selection
into 653cd50 on master.

[coveralls]

coverage: 96.154%. remained the same
when pulling 93b5b22 on add-postgis-udf-for-selection
into 653cd50 on master.