Support for PostgreSQL 9.1+ new Extensions model

[robe]

Since this feature was partly created with us in mind, we should take advantage of it. We might not get to it for PostGIS 2.0 though sicne I haven't looked at how much effort is involved.

So with the new way people would be spatially enabling their PostgreSQL database with

CREATE EXTENSION postgis VERSION '2.0';

This will also result in cleaner backups and restores since from what I can gather there are provisions to not backup functions etc. with data.

This will make a bunch of other things easier. People wanting to install in a postgis schema or migrate their existing to postgis schema.

ALTER EXTENSION postgis SET SCHEMA postgis;

And upgrading will be easier as well so to upgrade from an old 2.0.0 that wasn't packaged as an extension would be in theory as simple as.

CREATE EXTENSION postgis VERSION '2.0' FROM unpackaged;

Still unclear how our minor upgrade would work or if we would have to label our version VERSION '2.0.0' to allow people to upgrade easily.

ALTER EXTENSION postgis UPDATE TO '2.0.1'