Require PostgreSQL 9.1.x at least

We've avoided doing this for a long time, but the need to "CREATE
EXTENSION citext" finally tipped the scales. Anyway, by now 9.1.x
is everywhere, and even 9.2.x is widely used/packaged.

We can benefit greatly from 9.1 performance improvements in some
cases, and we can also drop some 8.x-specific things we do.

People still running 8.4 can use pg_upgrade to switch to 9.x; those
running older versions will need to pg_dump/restore their database.