Add support for PostgresSQL set_notice_receiver

We only add the support for it when using the pg gem.
Add test for set_notice_receiver
Merge branch 'pg_notice_receiver' of git://github.com/jeltz/sequel
Refactor and add documentation for notice_receiver commit

Shuffle some code around in Database#connect in the postgres
adapter so that you don't need to check for pg multiple times.

Change the spec to test that the notice receiver actually works.