Install PostgreSQL 9.2 via SCL

Just enough changes to RPM spec so that it installs correctly and

    scl enable postgresql92 'psql --version'

works.

Further work required to create a postgresql-9.2 cartridge on RHEL.
Leave versions/9.2 intact
Provide postgresql-9.2 formally

The cartridge is still horribly broken.
Add lib/postgresql_context to allow execution of SCL-based PostgreSQL commands

The structure is very much like that of the Ruby cartridge.

At this point, at least with ruby-1.9, the postgresql-9.2 cartridge can
be added without errors.
Add 'update-configuration' to set up environment variables so that shell process has access to 9.2 binaries
Tweak psql() to use the correct binary
Turn on PostgreSQL 9.2 testing