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.